Self-correcting self-checking turbine meter

ABSTRACT

A turbine meter is disclosed in which a sensing rotor downstream from the metering rotor senses changes in the exit angle of the fluid leaving the metering rotor, the output from the sensing rotor being combined with the output from the metering rotor to produce a corrected output indicative of the flow of fluid through the meter. The output from the sensing rotor and the output from the metering rotor may be compared to provide an indication of deviation from performance at calibration.

FIELD OF INVENTION

This invention relates to turbine meters of the type shown in U.S. Pat.No. 3,733,910 and is particularly concerned with apparatus and methodsof ascertaining and maintaining the accuracy of such turbine type flowmeters.

BACKGROUND OF THE INVENTION

Turbine type flow meters have been used for many years in themeasurement of fluids and this type of metering has become increasinglypopular because of its simplicity, repeatability, reliability and therelatively greater accuracy which turbine meters provide over otherforms of meters particularly at large quantities of flow.

It is generally understood in the art that each meter which ismanufactured and assembled in accordance with conventional methods hasits own unique registration or calibration curve. At the time ofmanufacture the actual flow through the meter is determined by a flowprover placed in a series in the test line with the meter beingcalibrated. A flow prover is a highly accurate instrument which itselfhas been calibrated to measure to a high degree of accuracy the quantityof flow. Meters produced by conventional manufacturing methods will eachshow a slightly different quantity of flow for the same quantity asshown by the flow prover. This is caused by a number of factors. Forexample, the different sets of bearings in one meter may impose aslightly different drag on the rotation of its rotor than the bearingsin other meters will impose on the rotors with which they areassociated. Also the angles at which the blades are oriented withrespect to the direction of fluid flow may vary slightly from meter tometer as will the area of annular flow passage through which the fluidflows as it passes through the meter. As a practical matter, it isimpossible under conventional production methods to maintain the effectof these factors precisely the same from meter to meter. Also, themechanical load imposed on the meter by the various drive elements suchas gears, magnetic coupling, and so forth between the rotor itself andthe registering mechanism will also vary from meter to meter. Thus,variations in these factors from meter to meter result in each meterhaving a unique value of flow through the meter for a given quantity offlow as measured by the prover. The ratio of the meter reading at anygiven rate of flow to the prover reading is referred to as the"percentage of registration." Thus, a meter which shows a registrationof flow of 999 cu. ft. of flow when the prover shows a flow quantity of1,000 cu. ft. is said to have a registration of 99.9%; that is, itregisters 99.9% of the fluid which actually flowed through the meter.The curve produced by plotting the percentage of registration of a meterat various rates of flow throughout its stated range of operation interms of flow rates is called the calibration curve and each meter hasessentially its own unique calibration curve.

In the field, therefore, if after a given period of time the meter showson its indicator a quantity of 10,000 cu. ft. of fluid having flowedthrough the meter at a given flow rate and if at that flow rate thepercentage of registration is 99.9%, the actual flow through the meteris 10,000 divided by 0.999 or 10,010 cu. ft. of fluid. As stated abovesince the calibration curve shows the percent of registration for thevarious flow rates throughout the operating range of the meter, bydividing the value shown on the meter register by the percentage ofregistration as shown on the calibration curve, for that meter at theflow rate the system was operating, the actual flow through the metercan be calculated.

In the course of the extended field use of the meter, any one or more ofthe factors mentioned above which influence the calibration curve canchange. For example, the rotor bearings may wear due to their continuoususe, resulting in much larger bearing friction than when they were new,foreign material in the fluid being metered can become lodged in thebearings, or the annual flow area may change because of the accumulationof foreign matter, causing a change in the influence which thoseparticular factors have on the amount the meter shows on its registerfor given amount actually passed through the meter. For example, if thebearing friction has increased due to continuous use to impart aconsiderably greater load on the rotor, then instead of registering99.9% registration in the example given above, the register on the metermay show only 98.9% of the fluid actually passed through the meter. Insuch a case the meter would register 1.1% less than 10,000 or 9.890 cu.ft. Since the operators have no indication that the meter is notoperating in accordance with its calibration curve, the reading of 9,890would be divided by the normal percentage of registration figure of99.9% which would give a spurious result of (9890/0.999)=9900 cu. ft.

In the past it has been the practice to periodically remove the meterfrom the line and to recheck it and recalibrate it against the standardof a meter prover. This, of course, requires considerable time andexpense and often results in the meter being operated while out ofcalibration for extended periods of time between calibration checks. InU.S. Pat. No. 4,091,653 assigned to the assignee of the presentinvention, a method and apparatus is disclosed for checking the accuracyand calibration of a turbine meter without removing the meter from theline and without the need of interrupting its normal service. Asdescribed in that patent, it has been found that changes in thecalibration or the percentage of registration of the meter result inchanges in the angle at which the fluid exits from the blades of themetering rotor. Thus, if at the time of original calibration the exitangle of the fluid leaving the rotor is noted and specified, byperiodically checking the exit angle of the fluid while the meter is inservice any deviations in the exit angle of the fluid from thatspecified at the time of original calibration will indicate to theoperator that the meter calibration has changed. That patent disclosedmeans provided within the meter to provide an indication of the exitangle of the fluid. The instant invention is an improvement to theinvention disclosed in that patent and provides a means of continuouslymonitoring the exit angle of the fluid so that when changes in the exitangle are sensed these changes are used to correct the registeredquantity of fluid in accordance with such changes to provide acontinuous and accurate registration of the flow thru the meter.

Prior attempts to achieve high accuracy in turbine meters are shown inthe U.S. Pat. No. to Souriau 3,142,179 and the U.S. Pat. No. to Griffo3,934,473. The patent to Souriau discloses a turbine meter in which thefluid entering the meter is given a tangential velocity by means offixed angularly oriented vanes. The fluid which then has a tangentialvelocity component impinges on the vanes of a metering rotor causing itto rotate. According to the teachings of that patent the meter operatesat greatly enhanced accuracy when the tangential velocity component iscompletely removed by the metering rotor. A brake is provided which isadapted to apply a braking torque to the metering rotor the magnitude ofthe torque being adjustable by rotation of a sensing rotor which isprovided downstream of the metering rotor. If the fluid leaving theblades of the metering rotor has any tangential velocity component leftwhich has not been removed by the metering rotor, the sensing rotor willbe caused to rotate. Rotation of the sensing rotor varies the amount ofbraking effort which is applied to the metering rotor until the meteringrotor is rotating at a speed at which all of the tangential velocitycomponent is removed from the fluid exiting from the blades of themetering rotor. In the present invention no tangential component isimparted to the fluid entering the metering rotor vanes and no attemptis made to remove the tangential component of velocity of the fluidleaving the metering rotor blades.

The patent to Griffo discloses a turbine meter in which a sensing rotordownstream from the metering rotor is adapted to rotate in a directionopposite from the direction of rotation of the metering rotor atapproximately the same speed as the metering rotor, the speed of thesensing rotor varying with changes in the speed of the metering rotor.In accordance with the invention disclosed herein it is shown to beadvantageous for the sensing rotor to operate in the same direction asthe metering rotor at a considerably reduced speed.

Other patents typical of efforts to enhance the accuracy of turbinemeters are the U.S. Pat. Nos. to Allen 3,241,366 and Hammond et al.3,710,622.

OBJECTS OF THE INVENTIONS

It is an object of this invention to provide novel apparatus and methodswhich are practical, simple, reliable and highly accurate within widerange of pressure and flow rates for continuously maintaining theaccuracy of a turbine type flow meter while the meter remains inservice.

It is also our object of this invention to provide novel means andmethods for continuously maintaining the corrected registration of thefluid flow through the meter and for continuously indicating the amountof deviation of the metering rotor registration from its calibration orany other reference value.

It is another object of this invention to provide means for continuouslymonitoring the exit angle of the fluid flow from the metering rotor andcorrecting the registered quantity of fluid flow in accordance with anychanges in the exit angle of the fluid to thereby provide an accurateregistration of the flow through the meter.

It is yet another object of the invention to provide apparatus by meansof which the accuracy of the registered amount of fluid passed throughthe meter is maintained while giving an indication of the amount ofdeparture of the exit angle from the initial calibrated value.

It is still another objective of the present invention to provide anovel method and apparatus which provides a measurement of a variablewhich may be compared with a reference value to determine whether theaccuracy of the meter has been changed and also its amount, and meansfor effecting a correction of the registered value in accordance withchanges in the variable.

It is still another object of this invention to provide an indicationwhen the meter is out of calibration and the amount of deviation fromcalibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a turbine meter, with a portion of the housingbroken away to show the measuring chamber and other details;

FIG. 2 is a longitudinal sectional view of the measuring chamber;

FIG. 3 is a diagram of an embodiment of a constant accuracy turbinemeter, using the flow direction-detecting pitot tube of U.S. Pat. No.4,091,653 as a sensing means;

FIG. 4 shows a diagram of another embodiment of a constant accuracyturbine meter;

FIGS. 5, 6A, 6B, 7A and 7B are velocity diagrams relating to the exitangle of fluid leaving the metering rotor and the sensing rotor to sensethis exit angle and to provide means to correct any change in exitangle, FIGS. 6B and 7B being respectively enlargements of the encircledportions of FIGS. 6A and 7A;

FIG. 8 is a section along 8--8 of FIG. 2;

FIG. 9 is the front panel of the electronic box of such a meter on whichthe various values, limits, etc., of the parameters involved in theinstant invention are displayed;

FIG. 10 shows the self-correcting circuit inside the panel of FIG. 9;

FIG. 11 shows the self-checking circuit inside the panel of FIG. 9;

FIG. 12 shows the relationship of the metering rotor speed to thesensing rotor speed for stated conditions throughout the rated range ofReynolds number for this meter;

FIG. 13 is a functional block diagram of the computer architectureimplementing a process in accordance with a further embodiment of thisinvention;

FIG. 14 illustrates a timing signal as developed within the system ofFIG. 13;

FIG. 15 shows a display board for providing a manifestation of fluidflow and for providing warning signals;

FIG. 16 is a more detailed functional block diagram of a portion of thesystem of FIG. 13;

FIGS. 17A, 17B and 17C together comprise a detailed schematic diagram ofthe system of FIG. 13; and

FIGS. 18A through 18F provide flow diagrams of the process as programmedin and executed by the system of FIGS. 13, 17A, 17B and 17C.

DESCRIPTION OF THE VARIOUS EMBODIMENTS

As disclosed in U.S. Pat. No. 4,091,653 the disclosure of which ishereby incorporated herein, changes in the angle at which the fluidflowing through the meter exits from the metering rotor (said angleherein designated θ ) are indicative of the changes in the meterregistration. In the invention of that patent, the exit angle was merelyindicated on a display to provide a basis for correcting the total flowthrough the meter as indicated on the meter register. FIG. 3 hereofshows a system whereby the exit angle is monitored and maintained at afixed value.

A flow direction-detecting pitot tube 12 similar to that disclosed inU.S. Pat. No. 4,091,653 is located downstream from its metering rotor20, as shown in said patent and in FIG. 3 hereof. At the time of initialcalibration the tube 12 is adjusted to a position commensurate with adesired exit angle θ and will therefore produce no output signal in theform of pressure differential Δp when the exit angle θ is at this value.When, however, in the course of service the exit angle θ deviates fromits value at initial calibration, the pitot tube will produce a pressuredifferential which varies with the amount of deviation Δθ. This pressuredifferential Δp which is representative of any deviation Δθ of the exitangle θ from its calibrated value θ* is impressed on differentialpressure transducer 14 as shown in FIG. 3. Transducer 14 converts thepressure differential Δp into an electronic error signal which variesdirectly with changes in the pressure differential and, therefore,changes Δθ in the exit angle. Thus,

    ΔpαΔθαError Signal

The deviation or error signal is then applied to a processor 16 where itis amplified and otherwise processed to condition it for application toa braking device 18. The braking device 18 functions to apply a brakingeffort to the metering rotor, the amount of which effort is determinedby the error signal input to the processor. Therefore, if in the courseof service the rate of rotation of the metering rotor 20 at a givenfluid flow rate is caused to slow down because of bearing wear or otherreasons, the exit angle θ of the fluid will increase which will causethe pitot tube 12 to apply a pressure differential which is sensed bytransducer 14 as a positive pressure. The output from the transducer 14and processor 16 which is representative of the change in the exit angleθ is applied to the braking device 18 which then functions to lessen thebraking effort applied to the metering rotor 20, resulting in anincrease in metering rotor speed and a decrease in exit angle θ. Theinitial adjustment in the braking force may not be sufficient to returnangle θ to its calibrated value. If not, Δp and the error signal fromthe transducer will persist causing the processor to make a series ofsuccessive adjustments. The meter 10 will again register the fluid flowaccurately within the limits of its original calibrated value. From theforegoing it will be appreciated that the braking device 18 mustfunction to apply a definite braking effort on the metering rotor 20 atall times even when the meter 10 is in calibration and operating withinpermissible limits of deviation in value of the exit angle θ from thecalibrated value θ*.

If for any reason the speed of the metering rotor 20 should increase fora given flow rate over its speed at calibration, the exit angle θ willdecrease which will cause the pitot tube 12 to apply pressuredifferential which is sensed by transducer 14 as a negative pressure,resulting in negative values of the outputs from the transducer 14causing processor 20 to decrease its output signal to cause the brakingdevice 18 to increase braking effort applied to the metering rotor 20,the speed of which will then be reduced to its original calibrated valueand the decreased exit angle will be nullified to yield zero errorsignal.

The foregoing describes an arrangement whereby the operation of aturbine meter 10 is adjusted in accordance with deviations in the speedof its metering rotor from its speed at the time of calibration so thatits output will always be accurate within the limits of its initialcalibration.

As described, deviations from calibrated operation are reflected inchanges in the exit angle θ of the fluid leaving the metering rod 20which changes are sensed by a flow direction-detecting pitot tube. Onedisadvantage in utilizing a pitot tube to sense changes in the exitangle is that the spaced openings and passages in the pitot tube asdescribed in U.S. Pat. No. 4,091,653 tend to become obstructed byforeign particles in the fluid being metered especially if the pitottube is to be left in the flow stream for continuous use.

It has been found that a second rotor 22 mounted for free rotation at aproper distance downstream of the metering rotor 20 may be used to sensechanges in the exit angle of the fluid leaving the metering rotor in amanner hereinafter described.

FIGS. 1, 2 and 8 show the internal details of a turbine meter 10 havingits sensing rotor 22 downstream of its metering rotor 20 to sense theexit angle θ of the fluid leaving the metering rotor 20. Turbine meter10 has a housing 50 with flanges 52 and 54 at the inlet and outlet ends,respectively, for connection into a fluid flow line. Upstream ofmeasuring chamber 58 is a flow guide 56 which is supported from housing50 by radially extending vanes 57. In addition to supporting guide 56,the vanes 57 serve to eliminate or minimize any tangential components inthe direction of fluid flow before it enters measuring chamber 58.Measuring chamber 58 is comprised of inner and outer concentriccylindrical walls 63 and 65 held together by spaced radial struts 114 toform annular passage 60, and is designed to fit into housing 50 in asuitable fluid-tight manner, so that all the fluid flows through theannular passage 60 (FIGS. 2 and 8) of the chamber. Inside measuringchamber 58, metering rotor 20 is mounted with radially-projecting blades62 completely spanning flow passage 60. Rotor 20 is fixed on shaft 64 bya key 66 and held in place by nut 68 and washer 70. An internal mountingmember 77 is comprised of transverse walls 77a and 77b being bridged bylongitudinally extending portions 77c and 77d. Walls 77a and 77b andbridging portions 77c and 77d are formed as one integral unit which issupported on wall 81 by any convenient means such as a series of screws83, and on wall 81 by a series of screws 83a. Walls 63 and 81 may beformed integrally and wall 81a secured to wall 63 by any convenientmeans such as screws, not shown. Bearing 72 is retained on shaft 64 by aportion of the hub of rotor 20, and bearing 74 is retained on the shaftby a nut 73. Bearing 74 is mounted in wall 77b and retained therein byretained plate 69 secured to the walls by screws. Internal walls 77a, 81and 81a form a chamber 71 and support the gear drive to the register 48and the rotation sensing apparatus, which will be described later.Openings (one of which is shown at 75) are provided with filters 75a,and provide pressure balance between the line fluid and the interior ofthe chamber 71, while the filters bar contaminants from the chamber.

The gear drive to the register 48 provides a mechanical read-out of theaccumulated volume of flow through the meter 10. It consists of a wormgear 76 fixed on rotor shaft 64 and meshing with and driving worm wheel78. Worm wheel 78 is fixed on an intermediate shaft 80 as by a pinthrough the hub 79 of worm wheel 78 and the intermediate shaft 80. Shaft80 is journalled in bearings 82 and 84 mounted respectively on bridgingportions 77d and 77c. One end of shaft 80 projects through bridgingportion 77c beyond bearing 84 and has pinion 86 mounted thereon. Pinion86 meshes with gear 88 mounted on shaft 90, which is rotatably mountedin the outer wall of measuring chamber 58 by a bearing 85 and by abearing (not shown) within the housing of register 48. As shaft 90rotates, it provides a direct mechanical drive through an assembly 92.(FIG. 1) comprised of a magnetic coupling and associated reduction gearsto drive register 48 mounted on top the meter housing. The magneticcoupling and associated reduction gears 92 are well known in the turbinemetering art, for example, see U.S. Pat. No. 3,858,488, issued Jan. 7,1975, and assigned to the assignee of this application.

In addition to the mechanical registration of flow, an electronic pickupassembly 100 is installed in the chamber 71. This assembly comprises aslot sensor 102 (FIG. 8) mounted on an internal wall of chamber 71, anda metal disc 104 having a number of radial slots 106 and mounted on therotor shaft 64 for rotation therewith. The sensor 102 is mounted toreceive a portion of disc 104 between two spaced portions of the sensor,so that, upon rotation of the disc the sensor detects the passage of theslots 106. A number of sensors are commercially available and the typeused in this embodiment are sold by R. B. Denison and is their model S J3, 5N. This type of sensor is supplied with a steady electric signal ofsay 40 K Hz. Alternate passage of slots and solid portions of the metaldisc between the spaced portions produce changes or modulations in theamplitude of the signal supplied to the sensor. These modulations arerectified or otherwise processed within the sensor to produce a pulseeach time the air gas is changed by passage of a slot between the spacedportions of the sensor. Conductors 108 (FIG. 2) extend from sensor 102to a source of power and to a processing circuit exterior of the meteras will be explained later.

Immediately downstream of the metering rotor 20, a thrust balancingplate 110 of proper diameter and axial length has a series ofcircumferentially spaced openings 112 which when the plate 110 is inposition are aligned with blades 62 of rotor 20 and blades 67 of sensingrotor 22 and are of the same radial dimension as annular passage 60 toproduce a continuation thereof. The portions of plate 110 radiallyinward are coextensive with the portions of rotors 20 and 22 which areradially inward of the blades 62 and 67. The peripheral portion of plate110 abuts a shoulder 120 in the housing of the measuring chamber and isheld in position by set screw 116.

Immediately downstream of thrust balancing plate 110 is a sensing rotorassembly 22 having blades 67. The construction is similar to themetering rotor assembly, except that the angle of the blades withrespect to the fluid flow is different and no provision for mechanicalregistration is necessary in connection with this rotor. A mountingmember 122 similar to mounting member 77 is comprised of walls 123 and124 which enclosed between them chamber 138. Rotor shaft 126 isjournalled on walls 123 and 124 by means of bearings 134 and 136 androtor 22 is secured on shaft 126 by means of key nut 132 and washer 130.The sensing rotor is thereby mounted for free rotation immediatelydownstream from metering rotor 20 and thrust balancing plate 110.

Within chamber 138, a pick up assembly 144 comprised of metal disc 148similar to disc 104, is mounted for rotation with shaft 126 and sensingrotor 22. A slot sensor 146 similar to sensor 102 has spaced armsembracing the disc as shown. Disc 148 has slots similar to disc 104 butnot the same number. Disc 148 and sensor 146 cooperate in the samemanner as disc 104 and sensor 102 to produce a pulse in conductor 150 inresponse to rotation on sensing rotor 22. Openings 140 and filters 142in walls 122, 123, and 124 provide pressure equalization between chamber138 and the flow passage of the meter.

Before entering the blades 62 of the metering rotor 20, the fluid isflowing in a direction of the vector V₁ parallel to the axis 23 ofrotation of the meter rotor 20, as shown in FIG. 5. As a result of itspassage through the blades 62 of the metering rotor 20, to overcomefluid and non-fluid drag, the direction and velocity of fluid flowleaving the rotor as indicated by vector V₂ is altered. The fluidflowing through the turbo-meter 10 approaches the rotor 20 as shown inFIG. 5, along a direction indicated by a vector V₁ striking the blades62 of the rotor 20 and exiting therefrom at an angle θ with respect to aline parallel to the axis about which the rotor 20 rotates. Therelationship between the various relevant parameters can be readilyunderstood by reference to velocity diagrams of rotor bladings of highsolidity design as shown in FIGS. 5-7B where:

β is the angle of inclination of the metering rotor blades with respectto the axis of rotation of the rotor 20;

θ is the fluid exit angle that is the angle by which the fluid isdetected from purely axial flow as a result of its passage through themetering rotor;

Va is the axial component of the absolute velocity V₁ of the fluidflowing through the meter and is equal to Q/A;

Q is the rate of flow of the fluid through the meter;

A is the effective area of the flow passage through the meter;

V₁ is a vector representing the direction and magnitude of the absolutefluid velocity as the fluid approaches the blade inlet section of therotor 20 and is assumed to be in a direction parallel to the rotor axisin which case V₁ =Va.

V₂ is a vector representing the direction and magnitude of the absolutefluid velocity as the fluid leaves the blades 62 of the meter rotor 20and as shown in FIGS. 5-7B, is offset from the axial direction by theangle θ i.e., the exit angle of the fluid;

U_(m) is a vector representing the direction and magnitude of the actualtangential velocity of the metering rotor 20. The vector U_(m) isparallel to a tangent of the circumference of the rotor 20 and is takenfrom a point displaced from the axis of the rotor rotation by aneffective radius r, which is calculated in accordance with the followingformula: ##EQU1## where r_(t) is the outside radius of the meter rotor20 and r_(r) is the radius to the inner roots of the rotor blades 62;

U_(i) is a vector representing the direction and magnitude of the idealnon-slip tangential velocity of the rotor 20 (at the effective radiusr). This quantity represents the velocity of a rotor not subject tomechanical loading such as bearing friction, the loading of the registermechanism and fluid friction.

ΔU_(m) is the difference between the ideal tangential velocity U_(i) andthe actual tangential velocity U_(m) of the meter rotor 20, due tobearing friction, fluid friction, and other loading.

γis the angle of inclination of the blades 67 of the sensing rotor 22with respect to the axis of rotation of the rotors 20 and 22;

U_(s) is a vector representing the direction and magnitude of thetangential velocity of the sensing rotor 22 at its effective radius asdefined in a manner similar to that as defined with respect to themetering rotor.

V₃ is a vector representing the direction and magnitude of the absolutevelocity of the fluid exiting from the blades 67 of the sensing rotor22.

Throughout this specification, quantities to which an asterisk * isappended represent their respective values at calibration.

As the fluid flowing through the meter 10 is a proper installationapproaches the blades 62 of the metering rotor 20, the direction offluid flow as indicated by vector V₁ is parallel to the axis of rotationof the rotors 20 and 22, that is, there is no significant tangentialcomponent in the direction of fluid flow. As the fluid impinges on theangular oriented blades 62 of the metering rotor 20, it exerts a drivingtorque on the blades 62 to cause the rotor 20 to rotate as itssynchronous speed corresponding to the given flow rate. Due to thefriction of the rotor bearings, fluid friction, the load imposed on therotor by the mechanical register and other factors, a resultingretarding torque is imposed on the rotor 22 which must be overcomebefore the rotor 22 can rotate at its synchronous speed. Therefore, thedirection of the fluid flow is deflected from its purely axial directionV₁ to V₂ as it passes through the blades 62 of the existing rotor 20.The amount of fluid flow is defected from its purely axial flow is theangle at which it leaves the metering rotor 20, at its exit section andis referred to as the exit angle θ. As shown the fluid is directed atthe sensing rotor 22 in a direction indicated by the vector V₂.

It will be understood from the foregoing and a reference to FIGS. 6A,6B, 7A and 7B that if the angle γ, that is, the angle of the sensingrotor blades, is equal to the exit angle θ, the sensing rotor 22 willnot rotate in either direction. In this situation, the direction offluid flow would not impart any rotational force to the sensing rotor22. If the exit angle θ is smaller than the sensing rotor angle asillustrated in FIGS. 7A and 7B, the sensing rotor 22 will rotate in thedirection indicated by the vector U_(s). It should be noted at thispoint that the angle at which the fluid enters the sensing rotor 22 willbe slightly less than the exit angle θ due to the momentum mixing effectwhen the fluid passes through the space between the two rotors and otherfactors. However, the difference is generally slight and the angle ofthe fluid entering the sensing rotor blading will be proportional to thefluid exit angle θ. Therefore, for purposes of the discussion herein,the angle of the fluid entering the sensing rotor blades will beconsidered to be the same as the exit angle θ of the fluid leaving themetering rotor.

FIG. 4 shows a system which like that of FIG. 3 applies a variablebraking force to the metering rotor 20 in response to variations in theexit angle θ of the fluid leaving the metering rotor 20 to therebymaintain the accuracy of the readout of the meter register. In thesystem of FIG. 4, however, the exit angle is sensed by a freelyrotatable sensing rotor 22 instead of a pitot tube. The internal designof the meter used in the system of FIG. 4 would be similar to that shownin FIG. 2 which was developed particularly for use in applicants'"self-checking" and "self-correcting" meter systems which will bedescribed in detail later on herein. However, in the FIG. 4 system asshown disc 104 is not utilized and sensing rotor utilizes a differenttype of an encoder disc 28 which replaces disc 148 of FIG. 2; also photodetectors or pick-ups are shown rather than the slot sensors shown anddescribed in connection with the design of FIG. 2.

The system shown in FIG. 4 will operate to impose a braking force on themetering rotor at all times, and the sensing rotor is designed to rotateat a low rate of speed alternately in opposite directions through a nullor stationary condition. FIGS. 6A and 6B show by vector representationthe effect of the flow of fluid through the metering and sensing rotors.In this system the calibrated values of the exit angle θ (θ*) will betheir average values when the meter is operating normally with somebraking force applied to the metering rotor which is determinedautomatically by the system as will be hereinafter described. Since theangle θ increases with load in the metering rotor, in order that thesensing rotor blade angle γ be approximately equal to the angle θ atcalibration (θ*), the angle γ is made slightly larger than thecalibrated value of angle θ would be if no braking force were applied tothe rotor.

If the value of γ* were to remain constant, and if the angle γ is thesame as θ* the sensing rotor would be stationary. However, if the speedof the metering rotor 20 decreases from its calibrated value the exitangle θ will increase and the sensing rotor 22 will be caused to rotatein one direction since θ>γ, while an increase in the speed of themetering rotor 20 will cause a decrease in the exit angle which willcause the sensing rotor 22 to rotate in the opposite direction sinceθ<γ. As seen in FIG. 6A, if the exit angle θ of the fluid flow exitingfrom the metering rotor 20 increases, the angle θ will be greater thanthe angle θ* and the fluid flow directed onto the blades 67 of thesensing rotor 22 will strike the right hand faces of the blades 67 asshown in FIG. 6A to cause the sensing rotor 22 to rotate to the left orin a counterclockwise direction viewed from the bottom of FIG. 6A.Conversely, if the rotational velocity of the meter rotor 20 shouldincrease, its exit angle θ will decrease and will be less than γ,whereby the fluid flow will strike the left hand faces of the blades 67of the sensing rotor 22, causing the rotor 22 to move to the right or ina clockwise direction viewed from the bottom of FIG. 6A. Rotation of thesensing rotor 22 is transmitted through shaft and gear connection 26 toan encoder disc 28, as shown in FIG. 4. A light source (not shown) ispositioned to direct a light beam through the openings of the encoderdisc 28 and onto a pair of photo detectors (not shown). This disc hastwo concentric series of openings about the axis of the disc, overlappedso that the light beam is periodically interrupted and the pair of photodetectors will produce pulses 30 and 32 for both clockwise andcounter-clockwise rotation of the sensing rotor. The concentric openingsare radially oriented in a manner to provide output pulses with a ±90phase difference with respect to each other. When the disc 28 isrotating in one direction the pulse signal 30 will lead pulse signal 32by 90° while rotation of the disc is the opposite direction will relsultin pulse signal 30 lagging pulse signal 32 by 90°. Thus the phaserelationship between the two pulse signals gives an indication of thedirection of rotation of the disc 28. The output from the photodetectors is supplied to a phase detector 34 which senses the phaserelationship between pulse signals 30 and 32 and therefore the directionof rotation of disc 28. The phase detector produces two digital outputsignals 35 and 37 which are applied to up/down binary counter 36. Theoutput on line 35 conditions the counter 36 to count either up or downdepending on the phase relationship between signals 30 and 32.

Depending on the phase relationship between signals 30 and 32 as sensedby phase detector 34, the up/down control signal applied via line 35will be such as to condition the counter 36 to count up or count downthe pulse values imposed on line 37 to the counter. As the sensing rotorrotates, line 37 applies the pulses from the photo detectors to counter36 which are counted up or down depending on the up/down signal receivedfrom the phase detector 34 which in turn depends on the direction ofrotation of the sensing rotor and disc 28.

A threshold and bias adjust logic circuit 38 contains elements wellknown in the art including (1) an analog/digital converter which takesthe analog value of the voltage out of buffer 46 as determined by thevalue of the bias in D/A buffer 40, and converts it to a digital value;(2) logic elements which apply offset values to the bias sensed by theD/A converter; these offset values establish plus and minus thresholdvalues for the bias; (3) a comparator which when instructed to do so bythe internal sequencing logic of circuit 38 will compare the pulse countvalue on counter 36 with the plus and minus threshold values todetermine whether or not the pulse count of counter 36 falls within oroutside the range established by the threshold values.

A timing circuit 41 causes the logic circuit 38 periodically at fixedintervals to perform the operations hereinafter described. At start upor initialization, by means of 15 manually operated thumbswitches thelogic circuit 38 is initially programmed with an initial bias factor.While this initial bias factor is arbitrarily selected, its generalvalue will be known from repeated experience. For illustrative purposesan initial bias factor having a value of 100 will be assumed. As soon asthe circuit 38 is programmed with the initial bias factor of 100 thisvalue is transferred to counter 36 and a signal is applied to D/A buffer40 which causes it to accept the value stored in counter 36. The D/Abuffer now contains the initial bias factor. This factor issimultaneously applied to D/A converter 44 which applies an analogsignal to buffer 46 corresponding to the initial bias factor. The buffer46 applies an output to brake 42 which causes an initial braking forcecorresponding to the initial bias factor of 100 to be applied to themetering rotor. Also, upon initial programming of logic circuit 38 itcomputes offset values to establish positive and negative thresholdvalues for the bias factor. For example it will be assumed that thelogic circuit 38 is programmed to apply an offset value of ±10 so thatthreshold values of 90 and 110 will be established.

Immediately upon the logic circuit 38 being programmed with the initialbias factor, it will signal the counter 36 to enable it to begincounting pulses from the sensing rotor. At the same time the timingcircuit 41 will be enabled to send timing pulses to the circuit 38defining fixed time intervals. During the first timing interval thecounter 36 will increment or decrement depending on which way thesensing rotor is rotating. In this example it will be assumed that theinitial bias factor loaded the metering rotor so that the sensing rotorwas caused to rotate in a direction in which the counter 36 isincremented. At the end of the first timing interval the timing circuitwill apply a signal to the logic circuit 38 which causes it toinstantaneously perform the following sequence of operations. Acomparison is made between the then existing value of the pulse count incounter 36 with the initially established threshold values of 90 and110. If the pulse count is outside the range of threshold values, say at115, the comparator in the logic circuit 38 signals the D/A buffer 40 toaccept the then existing pulse count on the counter 36 as the new biasfactor. The buffer 40 then sends a new signal to D/A converter 44 whichcauses it to produce a new analog signal to buffer 46 which in turnproduces a new output to the brake causing the braking force toincrease. The speed of the metering rotor is, therefore, decreased.

The A/D converter is logic circuit 36 now senses the value of the newoutput from buffer 46 (corresponding to bias factor 115) and converts itto digital form causing the logic circuit 38 to compute new thresholdvalues of 105 and 125. All of the functions of the logic circuit 38 forthe first timing interval have now been performed.

At the end of the second timing interval the pulse count on the counter36 will again be compared with the threshold values of 105 and 125. Ifthe pulse count value in counter 36 is within this range, nothinghappens until the end of some future timing interval when theaccumulated pulses on the counter 36 are outside the range. If the newbias factor and resulting increase in braking force was not yetsufficient to reverse the direction of rotation of the sensing rotor,counts will continue to be incremented on counter 36 during subsequenttiming intervals until the accumulated pulse count exceeds the upperthreshold value. When at the end of a subsequent interval the pulsecount on counter 36 exceeds 125, e.g. 126, a new bias factor of 126 withnew threshold values of 116 and 136 will be established which throughthe process described above will result in a slightly increased brakingforce on the metering rotor sufficient to cause the sensing rotor toreverse direction of rotation causing the phase relationship between thepulses trains 30 and 32 to reverse which causes pulses from the sensingrotor to decrement the pulse count on counter 36 from 126. This pulsecount will continue decrementing in succeeding timing intervals untilthe lower threshold value is exceeded. Thus, when the counter 36 isdecremented to something less than 116, e.g. 115, a new bias factor of115 together with new threshold limits of 105 and 125 are establishedwhich causes the braking force on the metering rotor to be decreased,increasing the speed of the metering rotor which causes the sensingrotor to again reverse so that the pulse counts from the sensing rotorwill again be incremented on counter 36. The pulses will be incrementeduntil the then existing upper threshold value of 125 is exceeded atwhich point the bias factor will again be established at the value inexcess of 125, e.g. 126. Thus in succeeding time intervals, bias factorsof 115 and 126 will be alternately established causing the sensing rotorto reserve direction each time a proper bias factor is estblished. Thiscauses the braking force on the metering rotor to be alternatelyincreased and decreased resulting in corresponding alternate decreasesand increases in metering rotor speed and successive reversals in thedirection of rotation of the sensing rotor. By this process an averagevalue of the metering rotor speed and exit angle θ is established whichmay be considered as their normal or calibrated values.

It will be understood that the drive or signal from the metering rotorto the register will be adjusted at time of calibration to register 100%registration as determined by a prover when the metering rotor andsensing rotor are operating at their normal or calibrated values.

If the average speed of the metering rotor is caused to change, whetherdue to changes in fluid flow rate, or malfunction of the metering rotor,new bias factors and threshold values will be established which willautomatically adjust the braking force on the metering rotor which willcause it to rotate at a speed which will produce 100% registration onregister 48.

The use of a sensing rotor 22 to sense the fluid exit angle θ from themetering rotor 20 provides a device much less likely to malfunction fromimpurities in the flow stream. It also provides a means of sensing theexit angle θ of fluid throughout the complete annular flow passage,providing a more accurate average exit angle reading than the singleflow direction-detecting pilot-tube could supply.

Both systems of FIG. 3 and 4 utilize a feed-back system and a brakingsystem of variable magnitude by means of which the braking magnitude onthe metering rotor 20 is altered in accordance with deviations in theexit angle θ from the sensing rotor blade angle γ to maintain the exitangle θ to have an average value equal to the sensing rotor blade angle(i.e., θ=θ*=γ), and thereby maintain the accuracy of meter registrationat its calibration value.

It has been discovered that the end results of constant accuracymetering by maintaining a constant fluid exit angle and a nulled sensingrotor by means of a braking system on the metering rotor 20 through afeed-back system can also be achieved in an alternative manner by anovel metering system consisting of simply a standard metering rotor 20and a free running sensing 22 rotor placed downstream, as shown in FIG.2 without the need of a braking device or feed-back system. Moreover,this metering system will not only perform "self-correcting" to maintainautomatically and continuously a constant meter accuracy at calibrationcondition, but it can also perform "self-checking" to indicateautomatically and continuously that the metering rotor is operatingeither within or without the selected deviation limit range from itscalibration meter registration as well as the magnitude of any suchdeviation. The basic concept of this novel metering system having this"self-correcting" and "self-checking" capabilities can be shown withreference to FIGS. 7A and 7B.

Noting the definitions of the vectors, angles and other parameters givenwith respect to FIGS. 7A and 7B, an expression may be developed for themeter registration of the metering rotor 20 that will provide a basisfor developing a self-correcting meter system that does not require theuse of the hysteresis brake 42 as shown in FIG. 4. First, the meterregistration of the metering rotor 20 is defined as the ratio of theactual tangential velocity U_(m) to the ideal tangential velocity U_(i)of the meter rotor 20, in accordance with the following expression:

    Meter Registration=Um/Ui                                   (1)

As seen from the velocity diagram of the exit velocity V₂ of the fluidflowing from the metering rotor 20 in FIG. 7, the actual tangentialvelocity U_(m) of the metering rotor 20 is the difference between theideal tangential velocity U_(i) and the meter rotor slip ΔUm due to thedrag or load placed upon the metering rotor.

Thus, equation 1 may be expressed as follows by simple substitution andrearrangement: ##EQU2##

Further, it is noted that if no loading is placed upon the meter rotor20, that the exit flow of fluid from the metering rotor 20 will be ofsubstantially the same magnitude as V₁ entering the meter rotor 20 andin a direction substantially parallel to the rotor axis, as indicated inFIG. 7A. The amount of drag or loading ΔU_(m) may be calculated usingthis vector diagram as follows: ##EQU3## Solving this equation forΔU_(m) provides the following equation:

    ΔUm=Va tan θ                                   (4)

Similarly, from FIG. 7A, the ideal tangential velocity U_(i) may beexpressed by the following expression: ##EQU4## Rearranging equation 5,the ideal velocity Ui may be expressed as follows:

    Ui=Va tan β                                           (6)

Substituting expressions 4 and 6 in expression (2) ##EQU5## It is seenfrom equation 7 that the change of actual rotor speed U_(m) of the rotor20 or meter registration (U_(m) /U_(i)) will result in a change of exitangle θ. If rotor speed U_(m) of the metering rotor decreases, the exitangle θ will increase and vice versa. It will therefore be evident thatin a conventional meter the meter registration (accuracy) will bedependent on and vary with, exit angle θ.

As will be hereinafter more fully examined in a practical embodiment ofthe invention herein described, it is desirable that the sensing rotorbe adapted to rotate in the same direction as the metering rotor but ata greatly reduced speed. As was explained in connection with the systemof FIG. 4 when the sensing rotor blade angle γ is the same as the exitangle θ the sensing rotor will be motionless. Thus by making the bladeangle γ slightly larger than exit angle θ, the sensing rotor will becaused to rotate in the same direction as the metering rotor but at agreatly reduced speed.

The meter registration of the sensing rotor 22 in terms of the idealrotor speed U_(i) of the metering rotor 20 for small blade angle γ ofthe blades 67 of the sensing rotor 22 and small angles of attack (γ-θ)of the fluid exiting from the meter rotor 20 and directed onto theblades 67 of the sensing rotor, will now be developed.

From FIGS. 7A and 7B it can be seen that the sensing rotor speed Us

    Us=Va tan γ-Va tan θ                           (8)

Therefore the registration of the sensing rotor in terms of the idealvelocity Ui of the metering rotor is ##EQU6## Substituting expression(6) into expression (9) becomes ##EQU7##

From expression (10) it is seen that any change in exit angle θ of themetering rotor 20 will change the speed of the sensing rotor 22. Anincrease of exit angle θ will decrease the sensing rotor speed U_(s). Inother words, as the exit angle θ becomes greater, the angle of attack ofthe fluid as it flows from the meter rotor 20 (as seen in FIG. 7A) ontothe blades 67 of the sensing rotor 22 becomes smaller, whereby the totalforce applied to the blade 67 becomes smaller. In case the exit angle θbecomes greater than the sensing rotor blade angle γ i.e., θ>γ, then tanθ> tan γ. Equation (10) shows the sensing rotor speed U_(s) becomesnegative if the angle θ were to increase above angle γ. Physically, thismeans the sensing rotor 22 would rotate in the opposite direction to thedirection as indicated by the vector U_(s) as shown in FIG. 7A i.e., thesensing rotor 22 is now rotating in the opposite direction of themetering rotor 20. Therefore, the above equation is valid for any amountof speed change of metering rotor 20 resulting in any amount of changein exit angle θ (θ could be greater or smaller than γ), and eitherdirection of rotation of the sensing rotor 22. However as will behereinafter explained, in practice, before this value of θ is reached tocause the sensing rotor to reverse direction of rotation, a signal willindicate that the meter is operating beyond the permissible limits ofdeviation from calibration so that the meter may be taken out ofservice.

From the above equations 7 and 10, it is seen that if the metering rotorregistration (U_(m) /U_(i)) changes, the exit angle θ will change, andthe sensing rotor registration (U_(s) /U_(i)) will also change. However,if we consider the difference U_(c) between the metering rotor speed orregistration and the sensing rotor speed or registration (sensing rotorspeed is taken as positive when it rotates in the same direction as themetering rotor 20, as shown in FIG. 7A, but negative when it rotates inthe opposite direction of the metering rotor), the following is derivedfrom equations (7) and (10): ##STR1## Equation 11 indicates that for afirst order of approximation, the difference in the rotor speed (orregistration) Uc/Ui between the metering rotor and sensing rotor,depends only on the metering rotor blade angle β and the sensing rotorblade angle γ, and therefore is a constant for a given meter employingthe invention thereof. It does not depend upon the varying load placedupon the meter rotor 20 or its exit angle θ. The physical reason forthis is that when the metering rotor speed Um changes for a given flowrate as a result of change in, for example, bearing friction and fluiddrag, the exit angle θ will have a corresponding change according toexpression 7. This change in θ will bring forth a corresponding changein sensing rotor speed U_(s) according to expression 10. It can be seenfrom expressions (10) and (11) that any amount of change in meteringrotor speed U_(m) produces a like amount of change in sensing rotorspeed U_(s) thus resulting in no net change in U_(c) if the differenceU_(c) between the metering rotor speed and the sensing rotor speed ismeasured as the basis of providing an improved self-correcting metersystem. In other words, the algebraic difference between the speed U_(m)of the metering rotor and the speed U_(s) of the sensing rotor willremain practically constant for all values of metering rotor speed at agiven flow rate, as long as the sensing rotor 22 is in its normaloperating condition. This relationship which is derived from expression(11) which provides the self-correcting feature of the instant inventioncan be expressed in terms of % of registration as

    Nc=Nm-Ns=constant                                          (12)

With the blades of the metering rotor 20 formed with an angle of 45°with the direction of the fluid flowing into the meter 10, as isconventional, the exit angle θ*, at calibration will be in order of twodegrees. The blades 67 of the sensing rotor 22 may be formed at an angleγ, which will cause it to normally rotate in the same direction as themetering rotor but at a much lower speed. In practical embodiment of theinstant invention the speed of the metering rotor 20 will be such thatthe metering rotor 20 will produce an output which is approximately 106%of the true flow through the meter as would be measured by a prover inseries in the test loop with the meter, the flow measured by the proverbeing considered to be 100% registration. The blades 67 of the sensingrotor 22 will be formed with an angle such that the sensing rotor 22will rotate in the same direction as the metering rotor 20 and its speedis such that its output represents approximately 6% of the true flow.The outputs from the metering rotor and sensing rotor may be consideredto be "offset" from the true or calibrated value of the flow through themeter. The relationship between the self-corrected % registration Nc andthe % registration of the metering rotor Nm and sensing rotor Ns isgiven by equation (12)

    Nc=Nm-Ns=106%-6%=100%

This relationship is also shown graphically by the solid lines of FIG.12 for all values of Reynolds number within the rated range of themeter. In the metering art, the performance of a meter is customarilyshown by plotting the percentage of registration shown by the meterversus Reynolds number. Reynolds number is a parameter which is wellknown in the art and represents a combination of the effects of thevelocity of fluid flow through the meter, the kinematic viscosity of thefluid, and the characteristic dimension of the meter being tested.

The validity of the relationship expressed in equation (12) may befurther demonstrated if it is assumed that the speed of the meteringrotor 20 is caused to decrease from its calibrated value (106%) to 105%registration. Such a decrease could be caused, for example, by bearingwear or foreign particles being lodged in the bearing of meter rotor 20.When this happens in a conventional meter the readout from the meterwould be less than its calibrated value and therefore less than theactual through-put through the meter. In the instant invention, however,the decrease of 1% registration of the metering rotor 20 will result inan increase in rotor slip ΔUm and therefore an increase in exit angle θof the metering rotor (tan θ/tan β increased by 1%=0.01 or θ increasedby 0.57° approximately) as seen in equation 7.

This increase in exit angle θ will reduce the angle of attack (γ-θ) ofthe sensing rotor by 0.57°, resulting in decrease in % registration bythe same amount (i.e. 1%) with the sensing rotor running at (6%-1%)=5%registration as observed from equation 10. The corrected % registrationNc remains unchanged according to equations 11 and 12 since

    N.sub.c =N.sub.m -N.sub.s =105%-5%×100%

This relationship between the % registration of the two rotors 20 and 22and to the corrected % registration remaining at 100% registration evenwhen the metering rotor is slowed down from 106% to 105% is showngraphically by the broken lines in FIG. 12.

Similarly, if the speed of the metering rotor increases from itscalibrated value for example to 107% at the same actual flow rate, theexit angle θ will be decreased by 0.57° (or tan θ/tan β will bedecreased by 0.01). This decrease in exit angle θ will increase theangle of attack (γ-θ) of the fluid onto the blades 67 of the sensingrotor 22, resulting in an increase of % registration of the sensingrotor 22 by the same amount, i.e. 1% from 6% to 7%. The corrected %registration Nc will still remain the same, i.e. 100% since

    N.sub.c =N.sub.m -N.sub.s =107%-7%=100%

Such a relationship is shown by the dotted lines in FIG. 12. Thus it isseen that a readout in terms of the algebraic difference between thespeed of the metering rotor 20 and the speed of the sensing rotor 22 ata given flow rate will provide a readout of 100% accuracy at allmetering rotor speeds even if the metering rotor speed departs from itscalibrated value, provided the sensing rotor 22 is functioning properly.It is this characteristic of the instant invention which is termed"self-correcting".

It will be appreciated that the designed speed of the sensing rotor 22could be any value relative to the designed speed of the metering rotor20 and the above expression for self-correction would still be true. Asa practical consideration, however, it is desirable to design thesensing rotor 22 to rotate at a much slower speed in comparison to thatof the metering rotor 20 to minimize the number of rotations and boththe radial and the thrust loading and thus wear on the sensing rotorbearings and thereby minimize the liklihood of sensing rotormalfunction. Also, as will be hereinafter demonstrated, it is desirablethat the speed of the sensing rotor be much less than that of themetering rotor in order to realize the full benefits of the instantinvention. In the embodiment described above the blades 67 of thesensing rotor 22 would be formed at approximately an angle of 3° to 4°(i.e. γ=3° to 4°) to provide a 6% registration at calibration whereasthe blade angle β of the metering rotor 20 is about 45° to provide 106%registration at calibration.

Also, the above expression is also valid for the case where the sensingrotor 22 is designed to rotate in the opposite direction from that ofthe metering rotor 20. In a meter in which the sensing rotor 22 isdesigned to rotate in the opposite direction from that of the meteringrotor 20 at calibrated speeds, the angle γ of the sensing rotor blades67 with respect to the direction of the flow of fluid into the meterwill be less than the exit angle θ and may even be negative with respectthereto; that is, diverging from the axis of rotation in a directionopposite from that of the exit angle θ. Therefore, a decrease in thespeed of the metering rotor 20 from its calibrated value which causes anincrease in the exit angle θ will cause an increase in the speed of thesensing rotor 22 and conversely an increase in the speed of the meteringrotor 22 over its calibrated value will cause a decrease in the speed ofthe sensing rotor. Thus, if the speed of the metering rotor 20 isrepresentative of 94% registration at calibration and the speed of thesensing rotor is 6% in the opposite direction of rotation of themetering rotor

    Nc=Nm-Ns=94%-(-6%)=100%

registration and a 1% decrease in metering rotor speed will cause a 1%increase in sensing rotor speed in the opposite direction so that

    Nc=93%-(-7%)=100%

Thus, the instant invention will provide a self-correcting capabilitywhen the rotors rotate in opposite direction as well as when they aredesigned to rotate in the same direction. However, when the two rotorsrotate in opposite directions the self-checking characteristic describedbelow is not as reliable as that with two rotors rotating in the samedirection as will be demonstrated hereinafter.

As indicated above, the self-correcting characteristic of the instantinvention will provide 100% registration at all speeds of the meteringrotor 20 at a given flow rate so long as the sensing rotor 22 isfunctioning properly. It would therefore be entirely possible for themetering rotor 20 to operate at speeds as low as 50% of its calibratedvalue and the corrected reading Nc would still provide accurateregistration. Thus the self-correcting feature provides no indication ofwhen either the metering rotor 20 or the sensing rotor 22 ismalfunctioning. In practice, in order to prevent excessive damage to themeter it is desirable that the meter be taken out of service andrepaired when the speed of the metering rotor deviates from thecalibrated value beyond certain prescribed limits.

The invention herein described and the importance of sensing the exitangle may be more fully understood from the following. Referring againto FIG. 5 the accuracy of a meter with no sensing rotor is equal to theratio of the actual velocity of the metering rotor Um to the idealvelocity of the metering rotor Ui which is the velocity it would attainif there were no resisting torque on the rotor. The meter accuracy (orregistration) is expressed mathematically in expressions (1), (2) and(7) above which for convenience are restated below. ##EQU8## From thisexpression it is evident that the meter accuracy is dependent on thevalue of exit angle θ. It is well known in the art that ##EQU9## WhereTn is the non fluid resisting torque acting on the metering rotor.

Tf is the resisting torque acting on the metering rotor due to thefluid.

(Tn+Tf)m is the total resisting torque acting on the metering rotor.

r is the effective radius of the rotor.

A is the effective flow area.

ρ is the fluid density.

and Q is the rate of fluid flow through the meter.

For small values of θ (normally approximately 3°) tan θ is approximatelyequal to θ. Therefore, ##EQU10##

Since the factor ##EQU11## is generally a small but variable quantity,the fluid exit angle θ in the conventional meter is therefore notconstant so that the meter accuracy expression ##EQU12## is notconstant. Since the only factors affecting meter accuracy are the angleθ and blade angle β, the blade angle being fixed, in a turbine meter inwhich the angle θ is held constant or one which operates independentlyof the angle θ, the meter accuracy will be constant. As described above,the meters depicted in FIGS. 3 and 4 achieve constant accuracy bymaintaining the exit angle θ constant, while the meters shown in FIGS.10 and 11 are independent of the exit angle θ. The manner in which thisis achieved by the instant invention may be more fully understood by thefollowing analysis.

Referring to FIG. 7A, since the thrust of the fluid on the sensing rotoris less than on the metering rotor (the angle being less than the angleβ), the bearing load on the sensing rotor is less than the bearing loadon the metering rotor and therefore the non fluid torque on the sensingrotor (Tn)_(s) is less than the non fluid torque on the metering rotor(Tn)_(m), i.e.

    (Tn).sub.s <(Tn).sub.m                                     (15)

The resisting torques due to fluid drag acting respectively on themetering rotor (T_(f))_(m) and on the sensing rotor (T_(f))_(s) act in atangential direction and are respectively proportionate to the sine ofmetering rotor blade angle β and the sine sensing rotor blade angle γ.Thus

    (T.sub.f).sub.m α sin β and (T.sub.f).sub.s α sin γ

However, because the relative velocity of the fluid exiting from thesensing rotor is less than the relative velocity of the fluid exitingfrom the metering rotor the ratio of their torques due to fluid (Tf)_(s)/(T_(f))m would be less than the ratio of sin γ/sin β. Thus ##EQU13##Therefore the ratios of the respective resulting torques due to fluiddrag is very much less than 1, ##EQU14## Since the non fluid torqueacting on the sensing rotor is less than that acting on the meteringrotor, and since the ratio of the fluid drag torque acting on thesensing rotor to that acting on the metering rotor is very much lessthan 1, it will be apparent that the total resisting torque acting onsensing rotor (Tn+Tf)s is very much less than the total resisting torqueacting on the metering rotor.

    (Tn+Tf)s<<(Tn+Tf)m                                         (18)

From expression (14) ##EQU15## From expressions (14), (18) and (19)##EQU16## It will therefore be seen that θs is very much smaller than θ.

The expression for meter accuracy (registration) for a meter employingthe instant invention in which both rotors rotate in the same directionis ##EQU17## which may be written ##EQU18## From expression (7),##EQU19## and from FIG. 7B Us=Va tan γ-Va tan (θ+θs).

Therefore expression (22) may be written ##EQU20## From FIG. 7A Ui=Vatan β and substituting in (23) the expression for accuracy for a meterin which both rotors rotate in the same direction is ##EQU21## Asdemonstrated above, θs is much smaller than θ and for all practicalpurposes may be disregarded so that ##EQU22##

Thus in a turbine meter employing the self-correcting feature of theinvention herein the variable fluid exit angle θ is replaced with aconstant rotor blade angle γ.

Through an analysis similar to that employed in the development ofexpression (24) it can be shown that the expression for the accuracy ofa meter in which the two rotors rotate in opposite directions is##EQU23## If in such a meter the sensing rotor is adapted to rotate atapproximately the same speed as the metering rotor, such as for example,as disclosed in Griffo U.S. Pat. No. 3,934,473, the blade angle γ of thesensing rotor will be essentially the same as the blade angle β of themetering rotor (the factor tan γ/tan β≈1) and expression (28) becomes:##EQU24##

It will be noted that the meter accuracy will vary with one half thevalue of the sensing rotor deflection angle θ_(s). Since in such a meterboth rotors rotate at approximately the same speed, the respectivedeflection angles will be approximately equal (θ_(s) ≈θ) and the amountof variation in registration would be one half as great as would beproduced in a conventional meter.

Again this is true only so long as the sensing rotor is notmalfunctioning and it should be pointed out that since the sensing rotoris rotating at approximately the same speed as the metering rotor thepossibility of the sensing rotor malfunctioning is of the same order ofmagnitude as that for the metering rotor.

For a meter in which the two rotors rotate in opposite directions butthe speed of the sensing rotor is, for example, one order of magnitudeless than that of the metering rotor, θ_(s) is small compared to θ orand can be disregarded. Expression (28) then becomes

    Meter Accuracy=1+(tan γ/tan β)                  (31)

Since the accuracy of such a meter is independent of any variablefactors, essentially complete correction and 100% registration will beachieved. However, as previously hereinabove noted, a meter in which therotors rotate in opposite directions will not provide a reliableindication of malfunction.

In the foregoing analyses, θ_(s) was regarded when the sensing rotorspeed is much less (e.g., one order of magnitude) than the speed of themetering rotor. It should be understood, however, that because of thefactor θ_(s) in expressions (23) and (28), the sensing rotor does inreality introduce a very small error into the meter accuracy orregistration. However, when the sensing rotor speed (and θ_(s)) is ofone order of magnitude less than the metering rotor speed (and θ) thedeviation from 100% accuracy caused by the sensing rotor is so small asto be within the accepted limits of measurable repeatability of themeter (±0.1%) and is therefore of no practical consequence.

It has been found that the ratio of the speed of the metering rotor 20to speed of the sensing rotor 22 provides a means to indicate if eitherthe metering rotor 20 or the sensing rotor 22, or both, aremalfunctioning. It will be understood, however, that in a meter in whichthe speed of the sensing rotor is significantly less than that of themetering rotor, as between the two rotors, any malfunction will probablybe due to metering rotor 20 because of the relatively higher radial andthrust loads as well as the higher speed at which it rotates as comparedto the sensing rotor 22.

In the embodiment described above where the initial, calibrated valuesof the metering rotor speed and sensing rotor speed are

    Nm*=106% and Ns*=6%

at 100% corrected registration the ratio of the metering rotor speed tothe sensing rotor speed is

    Nm/Ns=Nm*/Ns*=106/6=17.67

If it is desired to operate the metering rotor within ±1% ofregistration at its calibrated value. ##EQU25## Therefore, as long asthe ratio of the speed of the metering rotor 20 to the speed of thesensing rotor 22 is within the limits of 15.29 to 21, the speed of themetering rotor 20 will be within ±1% of its calibrated value. If,however, the speed of the metering rotor 20 should drop below theprescribed limits say 2% below its calibrated value, ##EQU26##Similarly, if the speed of the metering rotor should increase 2% aboveits calibrated value, ##EQU27##

Thus, by continuously monitoring the value of Nm/Ns, means is providedto sense a deviation of the speed of the metering rotor 20 from itscalibrated value beyond the prescribed limits, so long as the sensingrotor is functioning properly.

If on the other hand, in the unlikely case where the sensing rotorbegins to malfunction while the metering rotor is functioning properly,the ratio Nm/Ns will similarly fall beyond the prescribed limits of15.29 and 21. To illustrate, assume in the embodiment described above,that the speed of the sensing rotor 22 is 1% slower than it should bewhile the metering rotor 20 continues to operate at its calibrated valuethen ##EQU28##

If the speed of the sensing rotor 22 is 1% faster than it should bewhile the metering rotor 20 is operating at calibrated value then##EQU29## Thus, when the metering rotor 20 is operating within ±1% ofits calibrated value, the ratio Nm/Ns will be within its prescribedlimits and the corrected registration Nc will be within its prescribedlimits and the corrected registration Nc will be at 100% accuracy if thesensing rotor 22 is operating properly. However, a deviation of ±1% inthe speed of the sensing rotor 22 from its normal value will cause theNm/Ns to fall outside the prescribed limits even if the metering rotor20 is operating at its calibrated value. A system will hereinafter bedescribed which monitors the speed of both the metering rotor 20 and thesensing rotor 22, and provides an output indicative of the differencebetween the speed of the metering rotor 20 and the sensing rotor 22, thesystem also being adapted to provide an indication whenever the ratioNm/Ns falls outside of the limits for which the meter and system is setto operate. An observer is therefore alerted to the fact that either oneor both rotors have deviated from their calibrated speeds.

In the embodiments described above, it was assumed that the meteringrotor 20 had deviated from its calibrated value while the sensing rotor22 is operating in its normal condition. Although the possibility isremote, when the sensing rotor 22 rotates at a much lower speed than themetering rotor 20, it is still possible for the sensing rotor 22 to slowdown from its normal value due to its own increased bearing friction. Insuch cases the "limit exceeded" indicator may be actuated even thoughthe metering rotor 20 is operating within the prescribed limits ofdeviation.

To illustrate, in the embodiment described above, where the calibratedvalues of the speed of the metering rotor 20 and sensing rotor 22 areNm=106% and Ns=6%, assume that the metering rotor is running 0.5% slowand the sensing rotor is also running 0.5% slow from its normal value.

Since a decrease in speed in the metering rotor causes an increase inexit angle which results in a corresponding drop in the speed of thesensing rotor (0.5%) and since the sensing rotor is running 0.5% slowerthan it should, we have

    Nm=106-0.5=105.50 and Ns=(6-0.50)=0.50=5.00

and ##EQU30## In such a case, the limit exceeded indicator will beactuated even though the speed of the metering rotor was within theprescribed limits of ±1%.

Consider the case where both rotors are designed to rotate in the samedirection in normal operation and consider the most likely abnormalcondition where both the metering rotor 20 and the sensing rotor 22 aremalfunctioning and therefore rotating slower than normal due toincreased bearing friction on each rotor by the amounts of (ΔNm) and(ΔNs) respectively. Then the corrected meter registration Nc is nolonger of 100% accuracy but will have an error (ΔNc) equal to the amountof slow down ΔNs of the sensing rotor 22, namely

    ΔNc=ΔNs                                        (32)

If the limits of deviation from calibration conditions of this"self-checking" and "self-correcting" meter designated Δα be set at ±1%,it can be shown the limits Δα=±1% have been exceeded and the "limitexceeded" indication will be produced once the sum of the metering rotordeviation (ΔNm) and the sensing rotor error (ΔNs) reaches the set limitof 1%, in accordance with the following:

    -[(ΔNm)+(ΔNs)]≈-1%=Δα      (33)

where (ΔNm) and (ΔNs) are only numerical values.

From equation 12 it is seen that the corrected meter reading Nc=Nm-Nswill be 100% accurate as long as the sensing rotor 22 is operatingnormally (i.e. ΔNc=ΔNs=0). However, if the sensing rotor 22 is in error,the maximum possible error of the corrected meter registration, (ΔNc)maximum will not exceed the set limit of Δα since

    (ΔNc) max=(ΔNs) max=⊥Δα⊥-(ΔNm)≦⊥Δα.perp.                                                         (34)

Now consider the case where the sensing rotor 22 is designed to rotatein opposite direction from that of the metering rotor 20 and againconsider the abnormal condition where both the metering rotor 20 and thesensing rotor 22 may slow down due to increased bearing friction by theamount of (ΔNm) and (ΔNs) respectively. As in the previous case, thecorrected meter registration Nc is no longer of 100% accuracy but willhave an error (ΔNc) equal to the amount of slow down of the sensingrotor, namely

    ΔNc=ΔNs                                        (32)

If the limits of deviation from calibration Δα are set at ±1%, thelimits Δα=±1% will be exceeded when the difference between the sensingrotor slow down ΔNs and the metering rotor slow down ΔNs reaches the setlimit of ±1% approximately and this relationship is expressed asfollows:

    [(ΔNs)-(ΔNm)]≈Δα=±1% approximately (35)

From equations 32 and 35 it is seen that the corrected meter readingNc=Nm-Ns will be 100% accurate as long as the sensing rotor 22 isoperating normally (i.e., ΔNc=ΔNs=0), just like the previous case wherethe rotors rotate in the same direction. However, if the sensing rotor22 is in error (ΔNs≠0), the maximum possible error of the correctedmeter registration (ΔNc) max can exceed the set limit Δα=±1% withoutproducing an indication of error. For example, assume the metering rotor20 is 1% slow (ΔNm=1%), the sensing rotor 22 could slow down to say 1.5%resulting in an error of 1.5% slow down in the corrected meterregistration (ΔNc=ΔNs=1.5%) without producing an indication that the setlimit Δα=±1% has been exceeded since by equation (35)

    [(ΔNs)-(ΔNm)]=[1.5%-1%]=+0.5%<+1%=Δα

or still within the set limit Δα=±1% When the metering rotor speed hasdecreased by 1% it will take a decrease in the speed of the sensingrotor of at least 2% and thus resulting in at least a 2% meter error(ΔN_(c) =ΔN_(s) =2%) to indicate that the set limit of Δα=±1% has beenexceeded since

    [ΔNs-ΔNm]=[2%-1%]=±1%=Δα

From the above description it is clear that two rotors rotating in thesame direction at normal conditions is the preferred design for"self-checking" in case the sensing rotor 22 may also be in error due toabnormal conditions, even though the probability of such occurrence issmall.

From the foregoing analyses it may be concluded that a meter employing asensing rotor which rotates in the opposite direction from that of themetering rotor at a speed substantially the same as the metering rotorsuch as disclosed in the aforementioned patent to Griffo, will providesome improvement over the accuracy obtainable from conventional metersand that a meter in which the sensing rotor rotates at a significantlylower speed than that of the metering rotor will provide a still furtherimprovement in meter accuracy regardless of the relative direction ofrotation of the two rotors. However, a meter in which the two rotorsrotate in opposite directions does not provide a reliable indication ofmalfunction (self-checking). Therefore, optimum performance is achievedwhen the sensing rotor is designed to rotate in the same direction asthat of the metering rotor at a speed of one order of magnitude lessthan the speed of the metering rotor. It will be understood, however,that a meter in which the sensing rotor rotates at a significantly lowerspeed than that of the metering rotor is within the purview of theinvention described herein regardless of the relative direction ofrotation of the rotors.

It is a common practice in the turbine meter art to provide"straightening" vanes upstream from the metering rotor similar to vanes57 (FIG. 1) of the meter herein described to minimize any tangentialvelocity components in the direction of fluid flow before it enters theblades of the metering rotor. However, disturbances or obstructionsupstream of the meter may cause a "swirl" (impart a tangentialcomponent) in the fluid flowing into the meter which may not be entirelyremoved by the straightening vanes. Also, such disturbances may cause anon-uniform velocity distribution in the fluid flowing into the meter.In other words, the axial velocity of the fluid at various points of themeter inlet section may vary considerably and non-uniformly. Inconventional meters any such swirl or non-uniform velocity distributionin the fluid entering the metering rotor will adversely affect the meteraccuracy. Tests have established that a meter employing the inventiondescribed herein is relatively insensitive to such phenomena. In otherwords, the accuracy of a meter employing the instant invention is notadversely affected by any swirl or non-uniform velocity distribution inthe fluid entering the meter rotor.

The manner in which the outputs from the metering rotor and sensingrotor are processed to produce a corrected meter registration will nowbe described by reference to FIG. 10. In an embodiment where the speedof the metering rotor at calibration is found to produce a registrationof 105.3%, the speed of the sensing rotor produces 5.3% registration sothat by subtracting the sensing rotor output from the metering rotoroutput the difference is representative of 100% registration as shown byequation 12. The system shown in FIG. 10 counts the number of pulses Pmfrom the metering rotor as produced by sensor 102 for every 500 pulsesPs from the sensing rotor as produced by sensor 146. In this embodiment500 pulses form the sensing rotor is equivalent to 57.34 ft³ of fluidflow through the meter 10 at calibration. In FIG. 10 a sequencer 154includes logic elements adapted to provide a sequential ordering ofcommands to the various other elements of the system and a timingcircuit which provides timing pulses of a frequency in the order of 100KHz. The sampling interval is the time it takes for the counter 151 toaccumulate 500 pulses from sensor 146. At start-up all of the countersand latches are initialized and, therefore, contain no counts and haveno values at their respective outputs and the sequencer 154 is in itsinitial mode awaiting a signal from counter 151 signalling that thecounter has accumulated 500 pulses. As soon as the counter 151accumulates 500 pulse counts it sends a signal to the sequencer whichcauses the sequencer 40 to index to its second mode in which ittransfers the pulse counts on counters 151 and 155 to latches 157a and157b respectively. This is done by sending a transfer signal to thelatches 157a and 157b which conditions the latches to accept the pulsecount signals from the respective counters. This transfer signal alsocauses the sequencer to automatically index to its third mode by meansof feedback of the transfer signal to the sequencer. In its third modethe sequencer sends a reset to both counters 151 and 155 to reset themto their initial condition to accumulate more pulse counts from thesensors. The accumulation of 500 pulses in counter 151 takes arelatively long period of time compared to the time it takes the systemto process the signals from the counters and latches and, therefore, thesequencer will remain in its first mode a relatively long period of timecompared to the time it takes to index through its subsequent modes. Itwill be understood that the purpose of the latches is to accept andstore the counts from the sensors 102 and 146 at the end of each 500pulses from sensor 146 so that the counters may at the end of each suchinterval be immediately conditioned to begin counting a new series ofpulses from the sensors while the pulse counts accumulated during thepreceding sampling interval are being processed. Again the reset signalto the counters is fed back to the sequencer to automatically index thesequencer to its fourth mode.

In its fourth mode the sequencer sends a command signal to themultipliers 152 and 156 which conditions them to accept respectively thesignal values appearing at the outputs of latches 157a and 157b. Themultipliers then perform a process which multiplies the value of thesignals from the latches 157a and 157b respectively by scaling factorsKs and Km. These factors are programmable and represent the number ofpulses produced by the metering rotor and the sensing rotor respectivelyfor each cu. ft. of fluid passing through the meter at calibration whichfactors are determined for each meter individually at initialcalibration.

Upon completion of the multiplication process the multipliers send acompletion signal to the sequencer which causes it to index to its fifthor subtract mode. In this mode the sequencer sends a signal to thesubtractor 158 which conditions it to accept the binary signals from themultiplier. The subtractor then performs the process of subtracting thevalue of the signal for multiplier 152 from the value of the signal frommultiplier 156, upon completion of which process, the subtractor sends aprocess completed signal to the sequencer causing it to index to itssixth mode. The output from the subtractor is a binary signal andrepresents the number of cu. ft. passing through the meter during eachsampling interval of 500 pulses from the sensing rotor. In its sixthmode the sequencer signals the down counter 159 to accept the binaryoutput signal from subtractor 158. Again, the transfer signal is fedback to the sequencer causing it to automatically index to its seventhand final mode.

In its final or decrement mode the sequencer simultaneously signals thedown counter 159 and divide-by counter 161 to accept timing pulses fromthe timing circuit in the sequencer. For each timing pulse received bythe down counter it is decremented one pulse count. At the same time thedivide-by counter accepts pulses from the timing circuit so that foreach count by which the down counter is decremented the divide-bycounter receives and accumulates one pulse count. Thus, by this processthe pulse count impressed on the down counter from the subtractor istransferred to the divide-by counter.

For each 10,000 pulses received by the divide-by counter it produces 1pulse which is applied to the register 160 which causes it to incrementin units of 1 cu. ft. of volume. Thus, for each pulse received from thedivide-by counter (and for each 10,000 pulse counts by which the downcounter is decremented) the register 160 indicates an additional 1 cu.ft. of fluid as having been passed through the meter. After thedivide-by counter has produced one pulse for each even 10,000 timingpulses received it will receive and hold any remaining number of pulsesfrom the down counter less than 10,000 which remainder will be carriedover and added to the next series of pulses transferred from the downcounter. When the down counter is decremented to zero by the timingpulses, it sends a decrement completed signal to the sequencer whichcauses it to index to its initial mode thereby disabling the downcounter and divide-by counter from accepting any more timing pulses andreturning the system to its initial condition so that the entire processmay be repeated upon receipt of the next 500 pulses at counter 151.

In the embodiment herein described, the slotted disc 104 produces 4pulses for each revolution of the metering rotor and the slotted disc148 produces 7 pulses for each revolution of the sensing rotor. In suchan arrangement it can be shown that for each 500 pulses Ps produced bythe sensing rotor, the average number of pulses Pm produced by themetering rotor over a number of sampling intervals is given by theexpression ##EQU31## where 1.0103 is a meter constant which takes intoaccount the slight difference in the effective flow area between the tworotors and also the wake effect and fluid coupling effect between thetwo rotors and is generally close to unity. Its exact value is to bedetermined during calibration.

α*=the % adjustment or registration of the sensing rotor at calibration.

Δα=% deviation from calibration.

In this embodiment, calibration shows that the sensing rotorregistration is 5.3%. Therefore, the average number Pm of pulses fromthe metering rotor at calibration for each 500 pulses from the sensingrotor is determined by equation (36) for α*=5.3 and Δα=0 as ##EQU32##

It will be understood that the fractional number (5735.018) of pulses isan average value which would be obtained by averaging the actual numberof pulses received from the metering rotor over several successivesampling intervals and that the actual number of pulses received in anygiven sampling interval may vary several pulses above or below thisaverage value. As mentioned above, 500 pulses from the sensing rotorrepresents 57.34 ft³ of fluid flow through the meter at calibration;that is when Δα=0. Therefore, at calibration when 500 pulses have beencounted by counter 151, counter 155 will have accumulated an average of5735.018 pulses and, therefore, the signals appearing at the output ofcounter 155 and output of latch 157b will have an average value of5735.018 when the outputs from counter 151 and latch 157a have a valueof 500. The multipliers 156 and 152 multiply the signals from thelatches 157b and 157a respectively by factors Km and Ks. The rotorfactors Km and Ks are determined at the time of calibration andrepresent the cu. ft. of registration for the respective rotors for eachpulse produced by the rotors. The factor Km is found by multiplying theflow through the meter as shown by the prover (57.34 ft³) by a factor of1.053 (the registration of the metering rotor=105.3%) and dividing bythe number of pulses Pm from the metering rotor. ##EQU33##

As in the case of Km, sensing rotor factor Ks is found by multiplyingthe flow through the meter by a factor of 0.053 (the registration of thesensing rotor=5.3%) and dividing the pulses Ps from the sensing rotor.##EQU34## The signal from the latch 157b having an average value of5735.018 pulse counts is multiplied in multiplier 156 by Km to produce abinary output having an average value representing 60.378/ft³. Similarlythe signal from the latch 157a having a value of 500 pulse counts ismultiplied in multiplier 152 by Ks to produce a binary output having avalue representing 3.0390 ft³.

The signals from the multipliers 156 and 152 representing respectivelyvalues which average 60.378 ft³ and 3.039 ft² are applied to thesubtractor 158 which subtracts the latter from the former to produce abinary output having an average value representing 57.339 ft³. Thebinary output from the subtractor is applied to the down counter in suchform that 573390 timing pulses from the timing circuit will be requiredto decrement the down counter to zero. As explained above, the divide-bycounter 160 produces an output pulse for each 10,000 timing pulsesreceived by it and thus it will produce 570,000/10,000 or 57 pulses tothe electromechanical register 160 causing it to register 57 ft³ of flowthrough the meter. The remaining 3390 pulses will be retained by thedivide-by counter and will be added to the pulses transferred to it fromthe down counter during the next sampling interval. Through successivesampling intervals the net effect of the system will be to subtract theoutput of the sensing rotor from the output of the metering rotor toprovide an accurate indication of flow on the register 160. It will beunderstood that since register 160 increments in units of 1 ft³,fractional values of ft³ will be held for succeeding sampling intervals.

It should be noted that the signal from multiplier 156 representing ametering rotor registration of 105.3% and having an average value of60.378 ft³ and the signal from multiplier 152 representing 3.0390 ft³ or5.3% registration are processed by subtractor 158 in accordance withequation (12) so that

    Nc=60.378-3.039=57.339 (100% registration)

If in the course of service the speed of the metering rotor decreasessome amount below its calibrated value e.g. 2% to 103.3% registration,an increase in exit angle θ will result. This increase in the exit angleθ of the flow from the meter rotor 20 will cause the sensing rotor 22 todecrease its speed or registration Ns by 2% to 3.3% registration. If therate of fluid flow though the meter 10 remains constant it will take alonger time period for the sensing rotor to produce 500 pulses and as aresult more fluid will flow through the meter 10 while the sensing rotor22 is producing 500 pulses. This new increased amount of fluid flow maybe calculated by multiplying the at-calibration flow by the ratio of thesensing rotor registration at calibration (5.3%) to the new registration(3.3%)

    57.34×(5.3/3.3)=92.09

Therefore, when the metering rotor 20 slows down 2%, 92.09 ft³ of fluidwill flow through the meter for each 500 pulses from the sensing rotor22. Also, because it takes a longer time period for the sensing rotor toproduce 500 pulses Ps, the number of pulses Pm will be increased. Thenew average number of pulses Pm for 500 Ps may be calculated fromequation (36) in which Δα=-2% or from the expression

    Pm=Pm*×(Rm/Rm*)×(Rs*/Rs)                       (37)

where

Pm*=average number of pulses from metering rotor at calibration

Pm=new average number of pulses from metering rotor

Rm*=rate of metering rotor registration at calibration

Rm=new rate of registration of metering rotor

Rs*=rate of registration of sensing rotor at calibration

Rs=new rate of registration of sensing rotor substituting ##EQU35##Therefore, when the speed of the metering rotor 20 slows down from itscalibrated value by 2%, it will produce an average number of 9035.1pulses while the sensing rotor is producing 500 pulses.

Therefore, over several successive sampling intervals the pulse countfrom the latch 157b to the multiplier 156 will have an average value of9035.8 which when multiplied by Km will produce an output signal havingan average value of 95.129 ft³ which corresponds to 103.3% registrationwhile 92.09 ft³ of fluid actually flows through the meter. Since thesensing rotor still produces 500 pulses during this time interval, thesignal from the multiplier 152 still produces a signal representing3.039 ft³ which now corresponds to 3.3% registration. When the twosignals are processed by subtractor 158 to subtract the value of thesignal from the multiplier 152 from the value of the signal frommultiplier 156, the subtractor will produce an output signal having anaverage value of 92.09 which corresponds to 100% registration.

If the metering rotor is caused to run 2% faster than its calibratedvalue, by employing the same process described above, it will be foundthat while the sensing rotor is producing 500 pulses, 41.6297 ft³ offluid will pass through the meter and over several successive samplingintervals the pulse count from the latch 157b to multiplier 156 willhave an average value of 4242.85, which when multiplied by Km willproduce an average output signal representing 44.6687 ft³ whichcorresponds to 107.3% registration. The subtractor subtracts the signalfrom multiplier 152 which has a value of 3.0390 from the value of thesignal from multiplier 156 which has a value averaging 44.6687 ft³ toproduce an average output signal representing 41.6927 ft³ correspondingto 100% registration. Thus, it can be seen that by subtracting thevolume as represented by the number of revolutions of the sensing rotorfrom the volume as represented by the number of revolutions of themetering rotor the result will always be representative of 100%registration at all values of speed of the metering rotor so long asthere is no malfunction of the sensing rotor.

FIG. 11 shows a system for implementing the self-checking feature of theinvention. The pulses Pm from the metering rotor are fed throughamplifier 186 to counter 188 where they are counted to produce a digitaloutput which is applied to comparator 190. The pulses Ps from thesensing rotor are fed through amplifier 180 to counter 182. A bank ofthumbswitches 184 may be set to condition counter 182 to produce oneoutput pulse for a selected number of Ps pulses input into the counter182. In the embodiment described, the counter 182 is conditioned toproduce one output pulse for each 500 pulses Ps from the sensing rotor.The interval between two successive pulses from counter 182 define thesampling interval for the FIG. 11 system. During this sampling intervalthe counter 188 accumulates pulses Pm. Each pulse from the counter 182is used as an enabling signal to cause comparator 190 to compare thenumber of pulses in counter 188 against the upper and lower limitnumbers set by thumbswitches 192 and 194. Comparator 190 contains logicelements which upon completion of the comparison process cause thecounter 188 to be reset to zero and counter 182 to be reset to the valueset by thumbswitch 184 thereby initiating a new sampling interval.

Thumbswitches 192 and 194 are connected to the comparator 190 torespectively condition the comparator 190 to the selected upper andlower limits of accepted deviation in the actual number of pulses Pmfrom the calibrated value for each 500 pulses from the sensing rotor.FIG. 9 shows a display panel on which the corrected registration isshown at 196 and the selected upper limit as set by switches 192 isshown at 198 and the selected lower limit is shown at 200.

The relationship between the average number of pulses Pm from themetering rotor and the number of pulses Ps from the sensing rotor in theembodiment in which the metering rotor disc 104 produces 4 pulses foreach revolution and the sensing rotor disc 148 produces 7 pulses foreach revolution is expressed by the equation 17 given previously.Therefore, ##EQU36## In the embodiment described where at calibration

    α*=5.3% and Δα=0, and

for every 500 pulses Ps from the sensing rotor ##EQU37##

Thus, when the meter is functioning at calibrated values, for each pulseto the comparator 190 from counter 182, a binary signal will be appliedto the comparator 190 from counter 188 which is representative of 5735pulses Pm from the metering rotor. It will be understood that in thefollowing discussion relating to self-checking, the calculated pulsecount values and those shown in the table below have been rounded off totheir nearest whole number values.

If it is desired to operate the metering rotor within deviation limitsthe ±1%, substituting in the equation (36) when Δα=-1% ##EQU38## andwhen Δα=+1% ##EQU39##

Thus, if it is desired to operate the metering rotor within thedeviation limits of ±1%, the switches 192 and 194 will be set tocondition comparator 190 for 4870 pulses and 7002 pulses respectively.With the comparator 190 so conditioned, if the signal from counter 188sensed by the comparator 190 is indicative of a number of metering rotorpulses between the limits of 7002 and 4870 for each enabling pulse fromthe counter 182, the comparator 190 will produce an output signal to the"normal" indicator light 206 to indicate the metering rotor is operatingwithin the prescribed limits of accuracy. If the signal to thecomparator from counter 188 is indicative of more than 7002 pulses Pmfor each enabling pulse from the counter 182, the comparator 190 willproduce an output to "lower limit exceed" indicator light 204 toindicate that the speed of the metering rotor or the speed of thesensing rotor is more than 1% slower than their calibrated values orthat their combined deviation is more than 1% slower than theircalibrated values. If the signal to the comparator 190 from counter 188is indicative of less than 4870 pulses Pm for each enabling pulse fromthe counter 182, it will produce an output to "upper limit exceed"indicator light 202 to indicate that the speed of the metering rotor orthe speed of the sensing rotor is more than 1% faster than theircalibrated values or that their combined deviation is more than 1%faster than their calibrated values. Comparator 190 also contains acircuit which counts the number of successive comparisons for which thepulses Pm are outside of the prescribed limits and if this abnormalitypersists for a given number of comparisons, for example 15, thecomparator 190 will produce an output to "abnormal" indicator light 208to indicate that the abnormality in operation is not a transientcondition.

It is important to note that by designing the sensing rotor 22 to rotateat a much lower speed (generally one order of magnitude less) than thatof the metering rotor and thus also resulting in even much lower thrustload on the sensing rotor bearings than on the metering rotor bearings,the sensing rotor 22 generally has much less chance of malfunction thanthe metering rotor 20. Therefore, when the "out of limit" indicatorlights turn on, it most likely means that the metering rotor isoperating beyond the chosen limit but the corrected meter readingNc=Nm-Ns remains at calibration value or 100% accuracy.

Below is a chart with Ps=500 pulses showing the upper and lower meteringrotor pulse limits for all values of deviation between 0 and ±4.00% forthe embodiment described above where the registration at calibration ofthe sensing rotor is 5.3%. With such a chart, an operator can set anydesired limits of accuracy drawn by simply adjusting the setting ofswitches 192 and 194 to the pulse values shown for the desired limits ofaccuracy. Since the calibrated value of the sensing rotor speed willvary slightly with each meter a similar chart must be provided for eachmeter showing the pulse values for the range of accuracies peculiar tothe calibrated value of the sensing rotor speed for each meter.

    ______________________________________                                        Δα                                                                           Pm             Δα                                                                            Pm                                      ______________________________________                                        0          5735 = Pm                                                          -0.10      5840           +0.10       5634                                    -0.20      5949           +0.20       5537                                    -0.30      6062           +0.30       5443                                    -0.40      6180           +0.40       5353                                    -0.50      6302           +0.50       5265                                    -0.60      6430           +0.60       5181                                    -0.75      6633           +0.75       5060                                    -1.00      7002           +1.00       4870                                    -1.25      7416           +1.25       4696                                    -1.50      7885           +1.50       4534                                    -1.75      8420           +1.75       4384                                    -2.00      9036           +2.00       4243                                    -2.50      10598          +2.50       3989                                    -3.00      12839          +3.00       3766                                    -3.50      16325          +3.50       3569                                    -4.00      22493          +4.00       3392                                    ______________________________________                                    

It will be noted that the parenthetical portion of equation (36) isproportional to the ratio of the speeds of the two rotors as well as theratio of the pulses. Thus, when both rotors are operated at calibratedvalues, ##EQU40## Similarly, substituting in the parenthetical portionof equation (36) ##EQU41## Thus, it may be stated ##EQU42##

The foregoing description and the systems shown in FIGS. 10 and 11contemplate using a pre-selected number of pulses from the sensing rotorto define a time interval during which pulses from the metering rotorare counted the number of pulses from the metering rotor being combinedwith and/or compared to the pre-selected number of pulses from thesensing rotor to provide a corrected registration as well as anindication of deviation from calibration. It will be understood as analternative that a pre-selected number of pulses from the metering rotorcould be counted to define a time interval during which the pulses fromthe sensing rotor are counted, the pulses from the two rotors thus beingcombined and/or compared in accordance with the teachings herein. Also,it is possible to provide a real time clock in the system of FIG. 10 and11 and count the pulses produced from each rotor during a given timeinterval as defined by the clock. Such a system will hereinafter bedescribed with respect to FIGS. 13-18F.

As indicated in FIG. 13, the computer system 300 implements anembodiment of this invention in which a program is stored in a memory312 which uses constants stored in a programmable constant storage unit314 and is executed under the control of a processor 302 which may be ofthe type sold by assignee hereof under part designation R6502-11. Aclock circuit 310, the output of which is indicated in FIG. 14, appliesa series of pulses to provide the system clock to the processor 302.Input and output signals are directed into and out of the system 300 viaan input/output circuit 306. As further illustrated in FIG. 16, thevelocities of the meter rotor 20 and the sensing rotor 22 are sensedrespectively by slot detectors 102 and 146 to derive signals to beapplied via amplifiers 336 and 334 respectively to an inputcommunication circuit 338, as illustrated in FIG. 16 as part of theinput/output circuit 306. Both the memory 312 and programmable constantstorage unit 314 are coupled to the processor 302 via bus 308 (FIG. 13).The input/output circuit 306 also includes an output communicationcircuit 340 which is coupled via bus 304 to the processor 302 to provideoutput signals for variously energizing the display lights such as thecompute display light 324, the normal display light 326 and the abnormaldisplay light 328, as well as an electromechanical totalizer 322 wherebythe current total of the measured fluid is displayed. As illustrated inFIG. 16, the output communication circuit energizes a plurality ofdrivers 344, 346, 348 and 350 to respectively actuate the indicatingdevices 322, 328, 326 and 324. In addition, the output communicationcircuit 340 provides a signal via the output driver 342 to provide asignal indicative of the flow rate through the meter 10. The displayelements shown in FIG. 16 are mounted upon a display board 320 as shownin FIG. 15, whereby the totalizer 322 and the display lights 324, 326and 328 may be readily observed by an operator.

In FIGS. 17A, 17B and 17C, there is shown a more detailed functionalblock diagram of the computer system 300, with like numbers indicatinglike elements. The slot detectors 146 and 102 (FIG. 17C) are coupledrespectively to the terminals 1 and 2 and 3 and 4 whereby thecorresponding inputs are applied through amplifiers 336 and 334respectively, to level translators comprised essentially of transistorsQ1 and Q2. The level shifted outputs are taken from the collectors oftransistors Q1 and Q2 and applied along lines 304b and 304c to theinputs CA1 and CA2 of the input/output circuit 306 (FIG. 17B), which maybe of the type sold by the assignee hereof under part designationR6522-11. Outputs are derived from the pins 10, 11, 12 and 13 of theinput/output circuit 306 and applied via a group of lines collectivelyidentified by the reference numeral 304d to the drive array 380 (FIG.17C) to variously provide signals indicative of the totalized flow andthe presence of normal, abnormal and compute conditions respectively.Additionally, a digital representation of the analog self-checkingsignal is provided by the input/output circuit 306 on pins 2 thru 9collectively identified by the numeral 304f. Pins 11 to 13 of theinput/output circuit 306 are also connected as shown in FIG. 17C via thegroup of lines 304e to the buffer amplifiers 346, 348 and 350 forenergizing the indicating devices 324, 326 and 328. In addition, signalsare derived from the collectors of the transistors Q2 and Q1 and areapplied via the driver array 380 to provide signals indicative of therotation of the main and sensor rotors.

A power supply 376 is shown whereby ±5 volts derived from an externalsource of d.c. voltage is applied to the various elements of thecomputer system 300. In FIGS. 17A and 17B, two distinct memories aredisclosed. A first memory 312 comprises a pair of ROMs 364 and 366 iscoupled via the address bus 308 and data bus 308a to the microprocessor302. As illustrated, the most significant bits of the address bus fromthe processor 302 are applied to decoder 372, which during systemoperation and depending upon the status of those bits selects either ROM364 or 366 as the device from which a certain location is to be read.ROMs 364 and 366 may be of the type sold by assignee under partdesignation R2332. During the initial development stage of the systemEPROMs may be substituted form ROMs 364 and 366 whereby the program maybe initially programmed and then reprogrammed as changes areincorporated into the system 300. Additionally, a second memory 312' iscomprised of RAM elements 368 and 370 which are used as temporary datastorage and is coupled to the processor 302 via address bus 308 and databus 308a. The RAMs 368 and 370 which may be of the type sold by IntelCorporation under part designation P2114, are also addressed via theaddress decoder 372. In a manner similar to that used for ROM 364 and366 as previously described, the decoder 372 provides a chip selectsignal to the RAMs 368 and 370, which enables these circuits to respondto the address on bus 308.

A power-on reset circuit 374 as shown in FIG. 17A is responsive on theinitial application of the d.c. system power +5 volts, and produces apulse which is applied via line 304a to reset the processor 302, wherebyan initialization and power on routine is executed. A clock signal asillustrated in FIG. 14 is developed by the system clock circuit 310which comprises an oscillator 362 having a crystal element Z1oscillating at four MHz. The output of the oscillator 362 is divided bydivider 360 comprised of a pair of flip-flops before being applied tothe clock input of the processor 302 which further routes this clocksignal to the remainder of the circuit. The programmable constantsstorage unit 314 is shown in FIG. 17B connected via address bus 308 anddata bus 308a to memory 312 and processor 302 whereby a set of constantsas programmed therein may be entered into the system 300. Divider 360and storage unit 314 may be of the types sold by National SemiconductorCorporation under respective parts designations 74LS74 and DM8577N.Also, an analog circuit output indicative of and proportional to theerror output may be derived from the digital representation of theoutput signals designated 304f produced by the input/output circuit 306and appearing on pins 2 thru 9 thereof, in conjunction with the cascadecoupled transistors Q4 and Q3 by the analog to digital converter 306a.

Equation (12) may be rewritten in terms of metering rotor and sensingrotor pulses as follows:

    Vc=Pm/Km-Ps/Ks                                             (38)

where Vc is the corrected volume in cu. ft. flowing through this meterduring a given period of time; Pm and Ps are respectively the pulsesfrom metering rotor and sensing rotor accumulated during said period oftime and Km and Ks are respectively the meter and sensing rotor factorsin pulses per cu. ft. of flow through the meter which factors aredetermined, at the time of initial calibration. The system 300 operatesto sense and count the number of pulses Pm and Ps produced respectivelyby the metering rotor and sensing rotor, and to solve equation (38) toprovide an indication of corrected volume Vc.

The corrected volume calculation is performed at the conclusion of acontinuously occurring 1-second time base, said time base beingdetermined by a counting interval set by the timing signal (1 second)supplied by the system clock circuit 310. In turn, the calculatedcorrected volume Vc is applied repeatedly after each such 1-secondtiming interval to the electromechanical totalizer 322, whereby thevalues of flow are summed over a period of time to give a total amountof flow of the fluid through the meter 10 during that time. Furthermore,the computer system 300 is designed i.e., programmed, to implementvarious checks on the operation of the meter 10. For example, if thespeed of the metering rotor 20 significantly decreases from itscalibrated value beyond prescribed limits as hereinafter described, anerror or malfunction condition is noted. Typically, the sensing rotor 22is designed to rotate at a significantly slower speed (one order ofmagnitude less) than that of the metering rotor 20. Under suchconditions, it is normally expected that the bearing of the meteringrotor 20 will degrade before that of the sensing rotor 22, with theresult that the speed of the meter rotor 20 will significantly decreasefrom its calibrated value beyond the prescribed limits. In such an eventthe factor Pm/Km becomes less than the factor Ps/Ks. Thus to detect sucha condition, the system 300 periodically checks the magnitude of (Pm/Km)relative to the magnitude of (Ps/Ks). If (Pm/Km) is less than (Ps/Ks),then the adjusted volume Vc is given by the following equation:

    Vc=Ps/Ks                                                   (39)

The adjusted volume Vc as indicated by equation 39 is an approximationof the fluid flow. In addition, upon detecting the condition where Pm/Kmis less than Ps/Ks an error condition is indicated and the abnormaldisplay light 328 will be energized, as hereinafter described.

Further, self-checking is accomplished by determining the percentage ofdeviation Δα of the sensor rotor speed from its calibrated value inaccordance with the following equation 40 which may be derived fromequation (36) ##EQU43## The deviation of the sensor speed from itsinitially calibrated value is continually calculated. In theself-checking calculation, the system 300 senses a predetermined numberof pulses P_(m) from the meter rotor and when this number equals thepredetermined number, e.g. 25,000, corresponding to 50 seconds ofmaximum flow rate, equation 40 is solved and the calculated value of Δαis compared with limits ±Δαp as preset by programmable unit 314. If thepreset limits are exceeded, i.e., ⊥Δα⊥ greater than ⊥Δαp⊥ then the meteris operating outside the chosen error limits and the abnormal displaylight 328 will be periodically energized. If however, the value of ⊥Δα⊥is less than the preset limits ⊥Δαp⊥, then the meter 10 is operatingnormally and the normal display light 326 is energized.

The computer system 300 also has the capability of providing anindication of flow rate F in terms of frequency (Hz) in accordance withthe following equation: ##EQU44## where P_(m) is the meter rotor speedpulse rate in terms of pulses per hour, which in turn equals 3600P_(m/t) in seconds, where t is a sampling interval, e.g. one second, Qmax is the rated flow rate of the meter 10 in cu. ft. per hour, and fmaxis the desired maximum output frequency at maximum flow. The program asstored and implemented by the system 300 calculates the flow rate F inaccordance with equation 41 based on a pulse counting interval of t,e.g. one second, as determined by the clock signal derived from thesystem clock circuit 310. The flow rate signal is derived from theoutput terminal 16 of the output driver 380, as seen in FIG. 17C.

A still further check is made by the computer system 300 for determiningwhether a minimum flow condition exists below which the resolution ofthe system will not provide an accurate indication of flow, bydetermining if the frequency of the sensing pulses is less than 1 Hz andthe frequency of the metering rotor pulse rate is less than 2 Hz for agiven period of time, e.g. 1 minute. This represents a normal conditionand an indication of that condition is produced by system 300 as will behereinafter described. Additionally, if the metering rotor pulse rate isless than 2 Hz and the sensing rotor pulse rate is greater than 1 Hz fora continuous period of one minute, this condition is considered torepresent a stalled metering rotor condition an indication of which islikewise provided by system 300 as will be hereinafter described.

Thus, the computer system 300 operates to continually calculate theadjusted volume Va and flow rate F, and to continuously check variousconditions whereby an indication of a normal or abnormal operatingcondition is provided.

Referring now to FIG. 18A to 18F, there will be now described in termsof an illustrative flow diagram, the program as stored within thecomputer system 300 as generally illustrated in FIGS. 17A, 17B and 17Cand in particular within one of its memories 364 or 366. Referring firstto FIG. 18A, there is shown an executive program by which the computersystem 300 as illustrated in FIGS. 17A, 17B and 17C is "initialized" or"powered up" whenever the initial application of the +5 d.c. power issensed by the power on reset circuit 374. Proceeding first through thestarting point in step 400, step 402 is executed in order that theinput/output circuit 306 is conditioned and in particular that its inputand output ports are dedicated in terms of receiving and transmittingrespectively data and also are conditioned with respect to energizingthe appropriate one of the display lights 324, 326 and 328. Next, thememory RAMs 368 and 370 are cleared in step 404. Constants such as themeter factors Km and Ks and the scaling factors including fmax are movedin step 406 from the programmable storage units 314 to the RAMs 368 and370. In step 408, these constants are used to calculate the frequencyfactor which is a scaling factor used in steps 518 and 434 describedbelow to provide an indication of the flow rate from the output drive380 as seen in FIG. 17C. Next, a timer T2, not shown but included withinthe input/output circuit 306, is initialized to a certain value andallowed to run from pulses originating from the system clock 310 suchthat repetitive and accurately finitely spaced timing signals areproduced which when sensed by the processor 302 will serve as the eventwhich triggers the self-check calculations and various status checks ofthe meter operation. Specifically, the particular number of pulsesderived from the clock circuit 310 are counted in timer T2 in order todefine a timing interval, specifically 50 Msec (milliseconds) and theoccurrence of the completion of such interval is continuously counted bythe processor 302 for 20 periods using timer T3 as described below togenerate the 1 second time base necessary for the self-correctingcalculation and the no flow and stalled metering rotor checks describedherein. Since these above steps occur only when system power is firstapplied, the steps 400 to 410 may be considered an "initialization" or"power on" routine whereby the system as shown in FIGS. 17A, 17B and 17Care prepared to effect a monitoring process whereby the turbine meter 10as shown in FIGS. 1 and 2 is made self-correcting in the sense that theindicated output is corrected and self-checked and that various errorconditions are detected to provide a manifestation thereof by energizingselected ones of the display lights 324, 326 and 328.

Next, in step 412, the output of the timer T2 is counted by a 1 secondsoftware timer T3, not shown but located within one or the other of RAMs368 or 370 to determine whether 20×50 Msec. pulses have been countedi.e., whether one second has elapsed. If not, a further check is made ofthe timer T3 until that time at which the timer T3 indicates one secondhas expired. At that point, a self-checking computation is made as willbe explained later and in step 414, the compute display light 324 istoggled. If in the course of the calculations of either of theself-correcting or self-checking routines, as will be described, anabnormal flash flag is set, the abnormal display light 328 will betoggled (switched on and off) in step 418. If not, as decided in step416, the process moves through transition point 5 to step 420 of FIG.18B, wherein a 1 minute software time T4, not shown but also locatedwithin one of the RAMs 368 or 370, is tested to determine whether it hasbeen turned on by step 446 as described below. If it has, then the countstored in the software timer T4 is incremented by one (representing thepassage of 1 second). If the timer T4 has not been turned on, theprocess moves to step 426 wherein it is determined whether a calculateflag has been set to initiate the calculation of the corrected volume ofthe self-checking calculations or to merely continue pulse counting. Inthe particular embodiment described herein, the self-correctingcalculations of corrected volume Vc are performed each second, whereasthe self-checking calculations are performed upon the occurrence of25,000 meter rotor pulses Pm. If the calculate flag is not set, theprocess moves to step 428 wherein the Pm and Ps pulses as derivedrespectively from the rotor slot sensors 102 and 146 and which werecounted during the just completed 1 second time interval defined bytimer T3, are shifted from a first set of register Pmi and Psi(interrupt counting registers within which the pulses were initiallyinterrupt counted during the just completed 2 second interval) locatedwithin the RAM memories 368 and 370, to a second set of hold registersPmc and Psc (calculations registers) defined by specific addresses alsowithin the RAM memories 368 and 370. This second set of registers isused in all calculations, while the first set of registers is only usedfor temporary storage, whereby the counts stored therein may be readilyincremented during interrupt processing. Next, the calculate flag is setin step 430 and the process jumps to the main calculation subroutinesi.e., the self-checking and self-correcting routines as will beexplained. After performing one of the self-checking or self-correctingroutines, the program returns to the process as shown in FIGS. 18B,wherein the half period for the flow rate frequency output calculated bystep 518 in terms of a clock scaling factor, which is determined in partby the frequency factor calculated in step 408 and metering rotor pulsefrequency Pmf is applied to a programmable divider within theinput/output circuit 306 in order to provide a scaled output indicativeof the flow rate from terminal 16 of the output driver 380. Next, step436 checks whether any flags have been set which would change theenergized states of any of the indicator lights 324, 326 and 328.

As indicated in FIG. 18B, at step 432, there is a jump to the maincalculating subroutine now explained with respect to FIG. 18C. The maincalculating subroutine enters through step 440 to first reset by step442 the first noted set of registers Pmi and Psi of the RAM memories 368and 370, in preparation for receiving the next series of pulses P_(s)from the sensing rotor detector 146 and the pulses p_(m) from themetering rotor detector 102. In the next step, decision step 444, thepulses p_(m) as transferred to the hold register of the second set ofthe RAM memories 368 and 370 are examined to see whether the previouslyaccumulated pulse count P_(m) from the meter rotor is less than 2indicating that the speed of rotation of the meter rotor 20 has beengreatly reduced frm its calibrated value and if so, to set a 1 minuteflag to initiate a timing period (timer T₃) to determine by step 448whether the reduced speed condition of the meter rotor 20 continues forthe one minute period. Since the pulse accumulation interval has beenset to one second by timer T₃ via the counting of the recurrence oftwenty 50 Msec timing intervals produced by the input/output circuit 306in conjunction with the system clock 310 by timer T₂ the pulsesaccumulated from both the metering rotor sensors 102 and 146 during thisone second interval will equal the frequency of the respective rotorsignals. If the reduced speed condition of the metering rotor 20 doesnot continue for a full minute, the process moves to step 460 and if thecondition does persist for one minute, the process moves to step 450wherein it is determined whether the speed of the sensor rotor 22 asindicated by the pulse count P_(s) timed over the one second interval isin excess of a predetermined frequency e.g., 1 Hz. If the frequency ofthe sensor rotor pulses is not above this 1 Hz amount, therebyindicating in conjunction with a low meter rotor pulse frequency asdetermined in step 444, that the fluid flow through the turbine meter 10is below the minimum amount for which system 300 will provide adequateresolution, step 452 causes the normal display light 326 to beenergized, while maintaining de-energized the abnormal display light328. On the other hand, if the speed of the sensing rotor 22 is greaterthan 1 Hz indicating a stalled meter rotor 20, step 454 de-energizes thenormal display light 326 and energizes the abnormal display light 328,indicating a malfunction (stalled metering rotor) of the turbine meter10. If in step 444, it is determined that the meter rotor 20 is rotatingabove the predetermined minimum, the one minute flag is reset wherebythe one minute timer T₄ is reinitiated to commence timing a new period,in the event the meter rotor pulse frequency as determined by decisionstep 444 during a subsequent cycle of program execution becomes lessthan 1 Hz.

At this point in the process as shown in FIG. 18C, the initial check todetermine whether this system is operative or not has been made and theprocess now moves to calculate the corrected volume V_(c) in accordancewith equation 38 set out above. In particular, step 460 determineswhether both of the accumulated metering rotor pulses Pm or sensingrotor pulses Ps equals zero indicating that each of the metering andsensing rotors 20 and 22 are at a standstill and if so, the processexits via transfer point 3. If not, step 462 determines whether only themeter rotor pulses Pm equals zero and if so, step 464 sets a flagindicating that the meter rotor 20 is at a standstill indicating thatthere is no flow through the meter 10 which may result from a stalledmeter rotor 20 or perhaps a fault in the sensor 102 or in the systemleading from sensor of detector 102. If Pm does not equal zero asdetermined by step 462, an indication is provided that the meteringrotor 20 is rotating. If at that time the sensing rotor 22 is at astandstill, there are no sensing rotor pulses and the routine as shownin FIG. 18C is capable of short cutting the calculations of correctedvolume V_(c). First, in step 466, the value of P_(m) /K_(m) iscalculated to be used in a manner to be described later. Next, in step468, a decision is made as to whether the number of pulses P_(s) equalsto zero, i.e., there are no sensing rotor pulses, and if yes, the valueof P_(m) /K_(m) as calculated in step 466 is assigned by step 470 to bethe corrected volume V_(c), since the value of the factor Ps/Ks(equation 38) is zero for the condition where P_(s) equals zero. At thispoint, the routine exits via point 2, whereby certain steps ofcalculations as would otherwise be required will not be performed.Proceeding from step 468, step 472 calculates the value of P_(s) /K_(s).If in step 474, it is decided that there are no pulses derived from themetering rotor, i.e., P_(m) equals zero, then the value of P_(s) /K_(s)is assigned by step 476 as the value of the correct volume V_(c) andsimilarly, the routine exits via point 2 to the subroutine as shown inFIG. 18D, whereby certain steps in the process will not be performed andthus computing time may be reduced. If there are sensing rotor pulsesP_(s) as decided by 468 and if there are metering rotor pulses P_(m) asdecided by step 474, then step 474 branches via exit point 1 to thesubroutine as shown in FIG. 18D. In this latter case, it is thennecessary to proceed through the entire subroutine as shown in FIG. 18D;whereas if there are either no sensing rotor pulses or no metering rotorpulses, the routine exits via one of the exit points 2 to therebyeliminate a number of the calculating or processing steps as shown inFIG. 18D. As shown in FIG. 18C, this saving of calculation time isachieved in part by splitting up of the calculation of the values P_(m)/K_(m) and P_(s) /K_(s).

The exit points 1, 2 and 3 from the routine of FIG. 18C transfer theprocess to various points of the subroutine as shown in FIG. 18D. Ifboth metering and sensing rotor pulses are determined by steps 462 and468 to exist the process enters via transfer point 1 to step 500,wherein it is determined whether the factor P_(m) /K_(m) is less thanthe factor P_(s) /K_(s), and if not, the corrected volume V_(c) iscalculated in step 504 in accordance with equation 38. In a particularabnormal situation where the performance of the metering rotor isdegraded to the point where the factor P_(s) /K_(s) exceeds the factorP_(m) /K_(m) as determined by step 500, an approximation of the correctvolume V_(c) is made in step 502 wherein the previously calculated valueof P_(s) /K_(s) is assigned as the approximated value of V_(c). At thispoint in the process as shown in FIG. 18D, a value of V_(c) has beencalculated in either step 504 or 502, or one of steps 470 or 476 asshown in FIG. 18C.

It will now be understood that the process described above calculatesthe corrected volume of fluid V_(c) at the end of each 1-second intervalwhich was passed through the meter during that interval. If the value ofV_(c) for that interval is not sufficient to increment register 322,that value of V_(c) will be stored in RAMs 368 and 370 as remainder Rwhich will be added to the results of the V_(c) calculation performed atthe end of the next succeeding 1-second interval.

Now, it is necessary to determine whether the value of the totalcorrected volume including remainder R from the preceeding interval issufficient to increment the mechanical totalizer 322 as shown in FIG.15. If so, the electromechanical totalizer 322 will be incremented.First, by step 506, the remainder R, which is the left over fraction ofthe totalizer factor that may have existed at the conclusion of allincrementations of the totalizer 322 due to the previous correctedvolume calculations, is added to the newly calculated value of correctedvolume V_(c) that was calculated for the just completed interval of onesecond to produce the total volume R₁ to be compared with the totalizerfactor. The totalizer factor is the volume e.g., 10 cu. ft., that isnecessary to increment by one the electromechanical totalizer 322. Next,step 508 takes the integer I of the newly calculated value of R₁. Theinteger value I is then compared to see whether it is equal to orgreater than the totalizer factor, and if so, the number of increments Nof the electromechanical totalizer 322 is determined in step 512. Thenew remainder R which is stored for use in the immediately followingcorrected volume calculation is determined in step 514 as the differencebetween R₁ and N×I. If the volume as represented by the value of integerI is less than the totalizer factor then the newly calculated adjustedvolume R₁ is saved for use in the immediately following corrected volumecalculation by being stored in the RAM memories 368 and 370 in thelocation set aside for R. The process continues in step 518 (FIG. 18B)to calculate the new half period count which is a scaling factor that isapplied to the input/output circuit 306 through step 434 to produce thefrequency based flow rate output signal given by equation 41.

At this point, the process moves via transfer point 4 to theself-checking subroutine as shown in FIG. 18E, wherein the systemdetermines whether it is operating normally or abnormally and provides acorresponding indication by energizing the corresponding display lights324, 326 and 328. In steps 520 and 522, the meter and sensor pulsecounts Pm and Ps are continuously transferred from the first set of holdregisters Psi and Pmi into a further third set of storage registers, Psrand Pmr (pulse accumulation registers) respectively of the RAM memories368 and 370, and are accumulated with the previous contents of theseregisters until 25,000 meter rotor pulses have been counted. This thirdset of storage registers is necessary since several program samplingcycles are necessary for the 25,000 meter pulse count accumulation tooccur. In this regard, it is preferred to permit a relatively longperiod of time to occur between the self-checking calculations in thatthe accuracy of the self-checking calculations or steps is improved. Inan illustrative example, where the system 300 and in particular themicroprocessor 302 responds to the clock signal derived from the systemclock circuit 310 to perform a self-correcting calculation each second,the system as explained above counts 25,000 meter pulses which willrequire approximately 50 seconds at maximum flow rate. Thereafter, adetermination is made in step 524 whether the number of meter pulses Pmris greater than 25,000 and if so, the various self-checking calculationsare initiated to determine whether the meter system is operatingcorrectly. If 25,000 metering rotor pulses have not been accumulatedthen the process proceeds to step 526 where the calculate flag is resetand pulse counting in the Pm and Ps registers continues. Upon thedetection of the occurrence of the predetermined number e.g., 25,000meter pulses, the contents of the holding registers of the third set,Pmr and Psr and the process initiates the self-checking calculation,namely, solving the equation 40 given above for the deviation fromcalibrated conditions in terms of Δα as by step 528. Next, the deviationvalue Δα is compared to the initially programmed sub-limit Δαρ of theacceptable deviation value, and if within acceptable sub-limits, step532 energizes the normal display light 526 while deactivating theabnormal light 328. If the calculated deviation Δα is greater than thepredetermined value Δαρ, step 534 makes a further decision to determinewhether the deviation value Δα is greater or less than the limit (α*-1)and if less, step 538 de-energizes the normal light 326, while causingthe abnormal light 328 to flash on and off to indicate that the limithas not been exceeded but the Δαρ value has been exceeded. If the amountof deviation Δα is greater than the limit as determined by step 534,step 536 de-energizes the normal display light 326 while continuouslyenergizing the abnormal light 328 to indicate a more severe condition ofmeter failure. Use of the "flashing" condition is facilitated by the"flash" flag as given in step 538 the status of which is tested in step416 to physically cause the abnormal indicator 328 to toggle. Thereafterin step 540, the third set of holding registers for accumulating themetering rotor pulses Pmr and the sensing rotor pulses Psr are reset tozero, before reseting the calculate flag in step 542, and returning tothe entry location 412 of the overall executive program.

Referring now to FIG. 18F, there is shown a subroutine for allowing thesystem to accept and process any one of three possible interrupts. Atthe occurrence of an interrupt the process jumps from any instructionlocation in the entire program as shown in FIG. 18A through 18E to theentry point 650 of the interrupt processing routine. In step 652, afirst determination is made of whether an input pulse has been producedby the metering rotor encoder via input CA2 of the input/output device306. If a meter pulse has been generated, the register in RAM memories368 or 370 which has been set aside for the metering rotor pulses andpreviously referred to as Pmi is incremented by one in step 654 and asignal acknowledging same is sent to the input/output circuit 306 toreset the interrupt line associated with the input CA2 such that anysubsequent metering rotor pulse will be acknowledged and processed bythe system. Similarly, in step 658, a determination is made whether aninput is applied to the CA1 terminal of the input/output device 306 andif so, the sensing rotor pulse register Psi of the first set which iscontained in RAMs 368 and 370 is incremened by 1 and likewse anacknowledgment reset signal is sent to reset the interrupt lineassociated with the input CA1. Thereafter, the determination is made bystep 664 whether the timer T3 has completed its 50 Msec timing cycle andif so, the 1 second software timer T₂ which is tested by clock 412 isincremented one by step 666 before applying a reset signal to theinterrupt line associated with the timer T₃ so as to allow theoccurrence of the completion of the next 50 Msec timing cycle to besensed by the system. At the culmination of this interrupt processingroutine, the program returns to the next instruction following theinstruction immediately preceding the occurrence of the interrupt.

The foregoing describes a meter and implementing electronic system whichwill provide an indication of fluid flow through the meter which iscontinuously corrected to calibrated values even though the speed of themetering rotor has departed from its calibrated value, and whichprovides an indication when either the metering rotor speed or sensingrotor speed or both have deviated from calibrated values beyond pre-setlimits. It will be understood that the inventions described herein areequally useful in the metering of gaseous fluids as in the metering ofliquid fluids.

I claim:
 1. A turbine meter comprising a housing, a metering rotorhaving spaced blades and mounted for rotation in said housing inresponse to the flow of fluid through said meter, output means actuatedby said metering rotor to provide an output indicative of the fluid flowthrough said metering rotor, a sensing rotor downstream of said meteringrotor for sensing the exit angle of the fluid leaving the blades of saidmetering rotor, said sensing rotor having blades oriented at a discreteangle with respect to the axis of rotation of said sensing rotor andbeing adapted for normally continuous rotation at a speed substantiallyless than the speed of said metering rotor, means actuated by saidsensing rotor for modifying the output from said metering rotor inaccordance with changes in said exit angle.
 2. The turbine meter definedin claim 1 in which said sensing rotor is adapted to rotate in the samedirection as said metering rotor.
 3. The turbine meter defined in claim1 in which the speed of said sensing rotor is one order of magnitudeless than the speed of said metering rotor.
 4. The turbine meter definedin claim 3 in which said sensing rotor is adapted to rotate in the samedirection as said metering rotor.
 5. A turbine meter comprising ahousing, a metering rotor having spaced blades and mounted for rotationin said housing in response to the flow of fluid through said meter,means actuated by said metering rotor to provide a first signalrepresentative of the speed of said metering rotor, a sensing rotordownstream of said metering rotor, said sensing rotor having bladesoriented at a discrete angle with respect to the axis of rotation ofsaid sensing rotor and being adapted for normally continuous rotation ata speed substantially less than the speed of said metering rotor, meansactuated by said sensing rotor to provide a second signal representativeof said exit angle, means for combining the values of said signals toproduce an output indicative of the performance of said meter.
 6. Theturbine meter of claim 5 in which said last mentioned means comprisesmeans to subtract the value of the signal representative of said exitangle from the value of the signal representative of the speed of saidmetering rotor to provide an output indicative of the fluid flow throughthe meter.
 7. The turbine meter defined in claim 5 in which the speed ofsaid sensing rotor is one order of magnitude less than the speed of saidmetering rotor.
 8. A turbine metering system comprising a housing, ametering rotor having spaced blades and mounted for rotation in saidhousing in response to the flow of fluid through said meter, meansactuated by said metering rotor to provide a first output having a valuerepresentative of the speed of rotation of said metering rotor, asensing rotor downstream of said metering rotor for sensing the exitangle of the fluid leaving said metering rotor, said sensing rotorhaving blades oriented at a discrete angle with respect to the axis ofrotation of said sensing rotor and being adapted for normally continuousrotation at a speed substantially less than the speed of said meteringrotor, means actuated by said sensing rotor to provide a second outputhaving a value representative of the speed of rotation of said sensingrotor, means to make a comparison the values of said first and secondoutput.
 9. The turbine meter defined in claim 8 in which the speed ofrotation of said sensing rotor is one order of magnitude less than thespeed of said metering rotor.
 10. The turbine meter defined in claim 8together with means to compare the value of said comparison with apre-selected range of values and means to provide an output when thevalue of said comparison is not within said preselected range of values.11. The turbine meter of claim 10 together with means to adjust thelimits of said preselected range of values.
 12. A turbine metercomprising a housing, a metering rotor having spaced blades and mountedfor rotation in said housing in response to the flow of fluid throughsaid meter, means actuated by said metering rotor to provide a firstsignal representative of the speed of said metering rotor, a sensingrotor downstream of said metering rotor adapted for rotation in the samedirection as said metering rotor for sensing the exit angle of the fluidleaving said metering rotor, means actuated by said sensing rotor toprovide a second signal representative of said exit angle, means tosubtract the value of said second signal from the value of said firstsignal to produce an output representative of fluid flow through saidmeter.
 13. A turbine meter comprising a housing, metering rotor havingspaced blades and mounted for rotation in said housing in response tothe flow fo fluid through said meter, means actuated by said meteringrotor to provide a first signal representative of the speed of saidmetering rotor, a sensing rotor downstream of said metering rotoradapted for rotation in the same direction as said metering rotor forsensing the exit angle of the fluid leaving said metering rotor, meansactuated by said sensing rotor to provide a second signal representativeof said exit angle, means to make a comparison of the values of saidfirst and second signals and means to compare the value of saidcomparison with a preselected range of values and means to provide asignal when the value of said comparison is not within said preselectedrange of values.
 14. The turbine meter of claim 13 together with meansto adjust the limits of said pre-selected range of values.
 15. A turbinemetering system comprising a housing, a metering rotor having spacedblades and mounted for rotation in said housing in response to the flowof fluid through said meter, means actuated by said metering rotor toprovide a first output having a value representative of the speed ofrotation of said metering rotor, a sensing rotor downstream of saidmetering rotor mounted for normally continuous rotation in the samedirection as said metering rotor and at a speed which varies with thevalue of the exit angle of the fluid leaving said metering rotor, meansactuated by said sensing rotor to provide a second output having a valuerepresentative of the speed of rotation of said sensing rotor, means tosubtract the value of said second output from the value of said firstoutput, and means to produce a third output the value of which isrepresentative of the difference between the values of said second andfirst outputs.
 16. A turbine metering system comprising a housing, ametering rotor having spaced blades and mounted for rotation in saidhousing in response to the flow of fluid through said meter, saidactuated by said metering rotor to provide a first output having a valuerepresentative of the speed of rotation of said metering rotor, asensing rotor downstream of said metering rotor mounted for rotation inthe same direction as said metering rotor and at a speed which varieswith the value of the exit angle of the fluid leaving said meteringrotor, means actuated by said sensing rotor to provide a second outputhaving a value representative of the speed of rotation of said sensingrotor, means to make a comparison of values of said first and secondoutputs and means to compare the value of said comparison with apreselected range of values and means to provide an output when thevalue of said comparison is not within said preselected range of values.17. The turbine meter of claim 16 together with means to adjust thelimits of said pre-selected range.
 18. A turbine meter comprising ahousing, a metering rotor having spaced blades and mounted for rotationin said housing in response to the flow of fluid through said meter,means actuated by said metering rotor to provide a first outputrepresenting registration differing from 100% registration, sensingmeans downstream of said metering rotor for sensing the exit angle ofthe fluid leaving said metering rotor, means actuated by said sensingmeans provide a second output representative of the amount by which saidfirst output is representative of registration differing from 100%,means to subtract the value of said second output from the value of saidfirst output, and means to provide a third output representative of thedifference between the values of said first and second output.
 19. Theturbine meter defined in claim 18 in which said first output isrepresentative of registration greater than 100%.
 20. A turbine metercomprising a housing, a metering rotor having spaced blades and mountedfor rotation in said housing in response to the flow of fluid throughsaid meter, means actuated by said metering rotor to provide a firstoutput representative of registration in excess of 100% registration,sensing means downstream of said metering rotor for sensing the exitangle of the fluid leaving said metering rotor, means actuated by saidsensing means to provide a second output representative of the amount bywhich said first output is representative of registration in excess of100% registration, means to make a comparison of the values of saidfirst and second outputs.
 21. The turbine meter defined in claim 20together with means to compare the value of said comparison with apre-selected range of values and means to provide an output when thevalue of said comparison is not within said pre-selected range ofvalues.
 22. A turbine meter defined in claim 21 together with means toadjust the limits of said preselected range.
 23. A turbine metercomprising a housing, a metering rotor having spaced blades and mountedfor rotation in said housing in response to the flow of fluid throughsaid meter, first pulse producing means actuated by said metering rotorto produce a given number of pulses per revolution of said meteringrotor, a sensing rotor downstream of said metering rotor for sensing theexit angle of fluid leaving the blades of said metering rotor, secondpulse producing means actuated by said sensing rotor to produce a givennumber of pulses per revolutions of said sensing rotor, first countingmeans responsive to a preselected number of pulses from said secondpulse producing means, second counting means for counting the number ofpulses from said first pulse producing means during the time period ittakes for said first counting means to count said preselected number ofpulses from said second pulse producing means, means to produce anoutput representative of the number of pulses counted by said secondcounting means during said time period, and means to produce an outputrepresentative of said preselected number of pulses counted by saidfirst counting means.
 24. The turbine meter defined in claim 23 togetherwith means to subtract the value of said last mentioned output from thevalue of said first mentioned output.
 25. The turbine meter defined inclaim 23 together with means to make a comparison of the values of saidoutputs.
 26. A turbine meter comprising a housing, a metering rotorhaving spaced blades and mounted for rotation in said housing inresponse to the flow of fluid through said meter, first pulse producingmeans actuated by said metering rotor to produce a preselected number ofpulses for a given number of revolutions of said metering rotor, asensing rotor downstream of said metering rotor for sensing the exitangle of fluid leaving the blades of said metering rotor, second pulseproducing means actuated by said sensing rotor to produce a pre-selectednumber of pulses for a given number of revolutions of said sensingrotor, first counting means responsive to a pre-selected number ofpulses from said first pulse producing means, second counting meanscontrolled by said first counting means for counting the number ofpulses from said second pulse producing means during the time period ittakes said first counting means to count said pre-selected number ofpulses from said first pulse producing means, means to produce an outputrepresentative of the number of pulses counted by said second countingmeans during said time period, and means to produce an outputrepresentative of said pre-selected number of pulses counted by saidfirst counting means.
 27. The turbine meter defined in claim 26 togetherwith means to subtract the value of said last mentioned output from thevalue of said first mentioned output.
 28. The turbine meter defined inclaim 26 together with means to make a comparison of the values of saidoutputs.
 29. A turbine meter comprising a housing, a metering rotorhaving spaced blades and mounted for rotation in said housing inresponse to the flow of fluid through said meter, first pulse producingmeans actuated by said metering rotor to produce a pre-selected numberof pulses for a given number of revolutions of said metering rotor, asensing rotor downstream of said metering rotor for sensing the exitangle of fluid leaving the blades of said metering rotor, second pulseproducing means actuated by said sensing rotor to produce a pre-selectednumber of pulses for a given number of revolutions of said sensingrotor, first time controlled counting means for counting the number ofpulses received from said first pulse producing means during a giventime interval, second time controlled counting means for counting thenumber of pulses received from said second pulse producing means duringsaid time interval, means to produce an output representative of thenumber of pulses counted by said first time controlled counting meansduring said time interval, and means to produce an output representativeof the number of pulses counted by said second time controlled countingmeans during said time interval.
 30. The turbine meter defined in claim29 together with means to subtract the value of said last mentionedoutput, from the value of said first mentioned output.
 31. The turbinemeter defined in claim 29 together with means to make a comparison ofthe values of said outputs.
 32. A self-correcting metering system forproviding an accurate manifestation of fluid flow through a metercorrected to compensate for conditions of use, said system comprising:a.a metering rotor mounted for rotation in response to the flow of fluidthrough said meter; b. a sensing rotor disposed downstream of saidmetering rotor and mounted for rotation at a speed dependent upon thevalue of the exit angle of the fluid leaving said metering rotor; c.first means responsive to the rotation of said metering rotor forproviding a first output indicative of the rotational speed of saidmetering rotor; d. second means responsive to the rotation of saidsensing rotor for providing a second output indicative of the rotationalspeed of said sensing rotor; and e. processing means comprising meansresponsive to the first and second outputs for providing respectively afirst signal representative of fluid flow which differs from 100%registration of the volume of the fluid flowing through said meter and asecond signal representative of the amount that said first signaldiffers from 100% registration of the volume of the fluid flow throughsaid meter and means for obtaining the difference between the values ofsaid first and second signals to provide an accurate indication of thevolume of fluid flowing through said meter.
 33. The metering system asclaimed in claim 32, wherein said processing means comprises a digitalcomputer processor and memory means.
 34. The metering system as claimedin claim 33, wherein said first means is coupled to said metering rotorto provide a first series of pulses whose frequency is dependent uponthe speed of said metering rotor, and said second means is coupled tosaid sensing rotor for providing a second series of pulses whosefrequency is dependent upon the speed of said sensing rotor.
 35. Themetering system as claimed in claim 34, wherein said processing meansincludes means for counting each of said first and second series ofpulses for a given time interval to provide respectively first andsecond indications of the volume of fluid flow during said giveninterval.
 36. The metering system as claimed in claim 35, wherein saidmemory means stores a metering rotor factor dependent upon the number ofpulses produced by said metering rotor at calibration for each unit ofvolume passed through said meter, and a sensing rotor factor dependentupon the number of pulses produced by said sensing rotor at calibrationfor each unit of volume passed through said meter.
 37. The meteringsystem as claimed in claim 36, wherein said control means comprisesfirst means for processing said first signal with said metering rotorfactor to provide an offset manifestation of the volume of fluid flowthrough the meter for said given interval and for processing said secondsignal with said sensing rotor factor to provide a second offsetmanifestation of the volume of fluid flow through said meter for saidgiven interval.
 38. The metering system as claimed in claim 37, whereinsaid metering rotor factor is selected to provide a manifestation offluid flow in excess of the actual flow through said meter at initialcalibration, and the sensing rotor factor is selected to provide amanifestation of flow which is equal to the excess manifestation offluid flow provided by said metering rotor such that the actual flow atcalibration is equal to the difference between the values of themanifestations provided by said metering rotor and said sensing rotor.39. A self-checking metering system for providing an indication ofchange in rotor performance from an initially calibrated valuecomprising:(a) a metering rotor mounted for rotation in response to theflow of fluid through said meter; (b) a sensing rotor mounted downstreamof said metering rotor for rotation at a speed dependent upon the valueof the exit angle of the fluid leaving said metering rotor, said sensingrotor having blades oriented at a discrete angle with respect to theaxis of rotation of said sensing rotor; (c) first means responsive tothe rotation of said metering rotor for providing a first signalindicative of the rotational speed of said metering rotor; (d) secondmeans responsive to the rotation of said sensing rotor for providing asecond signal indicative of the rotational speed of sensing rotor; and(e) processing means including first processor means responsive to thefirst and second signals for obtaining a first ratio signal indicativeof the ratio of the current rotational speed of said metering rotor tothe current rotation speed of said sensing rotor, second processor meansfor providing a second ratio signal indicative of the ratio of therotational speed at calibration of said metering rotor and therotational speed at calibration of said sensing rotor, and a means forobtaining the difference between the first and second ratio signals toprovide an indication of the change in rotor performance betweencalibration and current operation.
 40. The self-checking metering systemas claimed in claim 37, wherein said first means generates the firstsignal as a series of pulses whose frequency is dependent upon therotational speed of said metering rotor and said second means generatesthe second signal as a series of pulses dependent upon the rotationalspeed of said sensing rotor.
 41. The self-checking metering apparatus asclaimed in claim 37, wherein said processing means includes countingmeans for counting each of the first and second series of pulses for agiven interval to provide a first count manifestation and a second countmanifestation, and means for processing each of said first and secondcount manifestations with a metering rotor factor and a sensing rotorfactor respectively to provide first and second volume manifestationsrespectively, said first means for obtaining a first ratio of the firstand second current volume manifestations and second means for obtaininga second ratio of the first and second volume manifestations atcalibration.
 42. The self-checking metering system as claimed in claim39, wherein said processing means further comprises storge means forstoring limit values of the difference between said first and secondratios and means for comparing the change in rotor performance with thestored limit values and if not within said stored limit values, means toprovide a manifestation thereof.
 43. A self-correcting and checkingmetering system comprising:(a) a meter rotor mounted for rotation inresponse to the flow of fluid through said meter; (b) a sensing rotordisposed downstream of said metering rotor and mounted for rotation at aspeed dependent upon the value of the exit angle of the fluid leavingsaid metering rotor; (c) first means responsive to the rotation of saidmetering rotor for providing a first series of pulses whose frequency isindicative of the rotational speed of said metering rotor; (d) secondmeans responsive to the rotation of said sensor rotor for providing asecond series of pulses whose frequency is indicative of the rotationalspeed of said sensing rotor; and (e) processing means comprising a clockfor generating a series of timing signals, the interval there betweendefining a counting interval; means for counting each of the first andsecond series of pulses for said intervals to provide respectively firstand second volume manifestations; storage means for storing a meteringrotor factor determined at calibration and a sensing rotor factordetermined at calibration; calculation means for periodically processingsaid first volume manifestation and said metering rotor factor toprovide a first calibrated offset manifestation of the volume of thefluid flow through said meter and for processing said second volumemanifestation and said sensing rotor factor to provide a secondcalibrated offset manifestation of the volume of the fluid flow throughsaid meter; first means for obtaining the difference between said firstand second offset manifestations to provide a manifestation of theactual volume of the fluid flowing through said meter; means responsiveto the first and second series of pulses for obtaining a first ratiosignal indicative of the ratio of the present rotational speed of saidmetering rotor to the present rotational speed of said sensing rotor;means responsive to the first and second series of pulses for obtaininga second ratio signal indicative of a ratio of the rotational speed atcalibration of said metering rotor and the rotational speed atcalibration of said sensing rotor; and second means for obtaining thedifference between the first and second ratio signals to provide anindication of the change of rotor performance between calibration andits present operation.
 44. The self-correcting and checking meteringsystem as claimed in claim 43, wherein said first difference means isoperative in response to each of the series of clock timing signals toeffect its calculation of the volume difference.
 45. A self-correctingand checking metering system as claimed in claim 43, wherein saidprocessing means further comprises means responsive to a predeterminednumber of pulses of said second series to initiate the operation of saidsecond difference means to obtain an indication of the change in theperformance of either or both.
 46. The self-correcting and checkingmetering system as claimed in claim 45, wherein said storage meansstores limit values of the difference between said first and secondratio signals and means for comparing the ratio difference with thestored limit values and if not within said stored limit values means toprovide a manifestation thereof.
 47. A turbine meter comprising ametering rotor having blades oriented to form an angle with respect tothe axis of rotation of said metering rotor, means actuated by saidmetering rotor for providing a first electric signal representative ofthe speed of said metering rotor, a sensing rotor downstream from saidmetering rotor for sensing the exit angle of the fluid leaving saidmetering rotor and having blades oriented to form an angle with respectto the axis of rotation of said sensing rotor, said last mentioned anglebeing substantially less than said first mentioned angle, means actuatedby said sensing rotor for providing a second electrical signalrepresentative of the speed of said sensing rotor, means for combiningthe values of said first signal and said second signal to produce athird electric signal representative of the value of fluid flow throughsaid meter.
 48. The turbine meter defined in claim 47 in which said lastmentioned means is comprised of means to subtract said second signalfrom said first signal.
 49. A turbine meter comprising a metering rotorhaving blades oriented to form an angle with respect to the axis ofrotation of said metering rotor for providing a first electric signalrepresentative of the speed of said metering rotor, a sensing rotordownstream from said metering rotor for sensing the exit angle of thefluid leaving said metering rotor and having blades oriented to form anangle with respect to the axis of rotation of said sensing rotor, saidlast mentioned angle being substantially less than said first mentionedangle, means actuated by said sensing rotor for providing a secondelectric signal representative of the speed of said sensing rotor, meansfor comparing the values of said first and second signals and means toproduce a third signal representative of the value of said comparison.50. The turbine meter defined in claim 49 in which said means forcomparing the value of said first and second signals is comprised ofmeans to take the ratio of the values of said first and second signals.51. A turbine meter comprising a housing, a metering rotor having spacedblades and mounted for rotation in said housing in response to the flowof fluid through said meter, means to minimize the tangential componentin the direction of fluid flow into said metering rotor, output meansactuated by said metering rotor to provide a first output representativeof the fluid flow through said metering rotor, sensing means downstreamof said metering rotor for sensing the exit angle of the fluid leavingthe blades of said rotor, said sensing means comprising a sensing rotoradapted to rotate in the same direction as said metering rotor atcalibration, means actuated by said sensing means to provide a secondoutput representative of the value of said exit angle and means tosubtract the value of said second output from the value of said firstoutput to produce a third output representative of the flow through saidmeter compensated for deviations in the performance of said meteringrotor from its performance at calibration.
 52. A turbine metercomprising a housing, a metering rotor having spaced blades and mountedfor rotation in said housing in response to the flow of fluid throughsaid meter, output means actuated by said metering rotor to provide anoutput indicative of the fluid flow through said metering rotor, asensing rotor downstream of said metering rotor for sensing the exitangle of the fluid leaving the blades of said metering rotor, saidsensing rotor being adapted for normally continuous rotation in the samedirection as said metering rotor and at a speed substantially less thanthe speed of said metering rotor, and means actuated by said sensingrotor for modifying the output from said metering rotor in accordancewith changes in said exit angle.
 53. The turbine meter defined in claim52 in which the speed of said sensing rotor is one order of magnitudeless than the speed of said metering rotor.
 54. A tubine metercomprising a housing, a metering rotor having spaced blades and mountedfor rotation in said housing in response to the flow of fluid throughsaid meter, means actuated by said metering rotor to provide a firstsignal representative of the speed of said metering rotor, a sensingrotor downstream of said metering rotor for sensing the exit angle ofthe fluid leaving said metering rotor, said sensing rotor being adaptedfor normally continuous rotation at a speed substantially less than thespeed of said metering rotor, means actuated by said sensing rotor toprovide a second signal representative of said exit angle and means tosubtract the value of the signal representative of said exit angle fromthe value of the signal representative of the speed of said meteringrotor to provide an output indicative of the fluid flow through themeter.